A-2 実装内容まとめ:

バックエンド: POST /api/plans/bulk_update/ API（field_ids, year, crop, variety を受けて一括設定）
フロントエンド: チェックボックス列、全選択/個別選択、一括操作バー（作物・品種セレクタ + 確認ダイアログ）

frontend/src/app/allocation/page.tsx

@@ -4,7 +4,7 @@ import { useState, useEffect, useMemo } from 'react';
 import { api } from '@/lib/api';
 import { Field, Crop, Plan } from '@/types';
 import Navbar from '@/components/Navbar';
-import { Menu, X, BarChart3, ChevronRight, ChevronDown, ArrowUp, ArrowDown, Copy, Settings, Trash2 } from 'lucide-react';
+import { Menu, X, BarChart3, ChevronRight, ChevronDown, ArrowUp, ArrowDown, Copy, Settings, Trash2, CheckSquare } from 'lucide-react';
 
 interface SummaryItem {
   cropId: number;
@@ -35,6 +35,10 @@ export default function AllocationPage() {
   const [newVarietyName, setNewVarietyName] = useState('');
   const [showVarietyManager, setShowVarietyManager] = useState(false);
   const [managerCropId, setManagerCropId] = useState<number | null>(null);
+  const [selectedFields, setSelectedFields] = useState<Set<number>>(new Set());
+  const [bulkCropId, setBulkCropId] = useState<number | 0>(0);
+  const [bulkVarietyId, setBulkVarietyId] = useState<number | 0>(0);
+  const [bulkUpdating, setBulkUpdating] = useState(false);
 
   useEffect(() => {
     fetchData();
@@ -321,6 +325,47 @@ export default function AllocationPage() {
     }
   };
 
+  const toggleFieldSelection = (fieldId: number) => {
+    setSelectedFields((prev) => {
+      const next = new Set(prev);
+      if (next.has(fieldId)) next.delete(fieldId);
+      else next.add(fieldId);
+      return next;
+    });
+  };
+
+  const toggleAllFields = () => {
+    if (selectedFields.size === sortedFields.length) {
+      setSelectedFields(new Set());
+    } else {
+      setSelectedFields(new Set(sortedFields.map((f) => f.id)));
+    }
+  };
+
+  const handleBulkUpdate = async () => {
+    if (selectedFields.size === 0 || !bulkCropId) return;
+    if (!confirm(`選択した${selectedFields.size}件の圃場に一括で作物・品種を設定します。\n既存の設定は上書きされます。実行しますか？`)) return;
+
+    setBulkUpdating(true);
+    try {
+      await api.post('/plans/bulk_update/', {
+        field_ids: Array.from(selectedFields),
+        year,
+        crop: bulkCropId,
+        variety: bulkVarietyId || null,
+      });
+      setSelectedFields(new Set());
+      setBulkCropId(0);
+      setBulkVarietyId(0);
+      await fetchData();
+    } catch (error) {
+      console.error('Bulk update failed:', error);
+      alert('一括更新に失敗しました');
+    } finally {
+      setBulkUpdating(false);
+    }
+  };
+
   const handleCopyFromPreviousYear = async () => {
     const fromYear = year - 1;
     if (!confirm(`${fromYear}年度の作付け計画を${year}年度にコピーします。\n既に設定済みの圃場はスキップされます。\n実行しますか？`)) return;
@@ -534,11 +579,63 @@ export default function AllocationPage() {
                   </p>
                 </div>
 
+                {selectedFields.size > 0 && (
+                  <div className="mb-4 p-3 bg-green-50 border border-green-200 rounded-md flex items-center gap-3 flex-wrap">
+                    <span className="text-sm font-medium text-green-800">
+                      <CheckSquare className="h-4 w-4 inline mr-1" />
+                      {selectedFields.size}件選択中
+                    </span>
+                    <select
+                      value={bulkCropId || ''}
+                      onChange={(e) => { setBulkCropId(parseInt(e.target.value) || 0); setBulkVarietyId(0); }}
+                      className="px-2 py-1.5 border border-gray-300 rounded-md text-sm focus:outline-none focus:ring-2 focus:ring-green-500"
+                    >
+                      <option value="">作物を選択</option>
+                      {crops.map((crop) => (
+                        <option key={crop.id} value={crop.id}>{crop.name}</option>
+                      ))}
+                    </select>
+                    {bulkCropId > 0 && (
+                      <select
+                        value={bulkVarietyId || ''}
+                        onChange={(e) => setBulkVarietyId(parseInt(e.target.value) || 0)}
+                        className="px-2 py-1.5 border border-gray-300 rounded-md text-sm focus:outline-none focus:ring-2 focus:ring-green-500"
+                      >
+                        <option value="">品種を選択</option>
+                        {getVarietiesForCrop(bulkCropId).map((v) => (
+                          <option key={v.id} value={v.id}>{v.name}</option>
+                        ))}
+                      </select>
+                    )}
+                    <button
+                      onClick={handleBulkUpdate}
+                      disabled={!bulkCropId || bulkUpdating}
+                      className="px-3 py-1.5 bg-green-600 text-white rounded-md text-sm hover:bg-green-700 disabled:opacity-50"
+                    >
+                      {bulkUpdating ? '更新中...' : '一括設定'}
+                    </button>
+                    <button
+                      onClick={() => setSelectedFields(new Set())}
+                      className="px-3 py-1.5 text-gray-600 hover:text-gray-800 text-sm"
+                    >
+                      選択解除
+                    </button>
+                  </div>
+                )}
+
                 <div className="bg-white rounded-lg shadow overflow-hidden">
                   <div className="overflow-x-auto">
                     <table className="min-w-full divide-y divide-gray-200">
                       <thead className="bg-gray-50">
                         <tr>
+                          <th className="px-2 py-3 w-10">
+                            <input
+                              type="checkbox"
+                              checked={selectedFields.size === sortedFields.length && sortedFields.length > 0}
+                              onChange={toggleAllFields}
+                              className="rounded border-gray-300 text-green-600 focus:ring-green-500"
+                            />
+                          </th>
                           {sortType === 'custom' && (
                             <th className="px-2 py-3 text-left text-xs font-medium text-gray-500 uppercase tracking-wider w-20">
                               順序
@@ -571,7 +668,15 @@ export default function AllocationPage() {
                           const selectedVarietyId = plan?.variety || 0;
 
                           return (
-                            <tr key={field.id} className="hover:bg-gray-50">
+                            <tr key={field.id} className={`hover:bg-gray-50 ${selectedFields.has(field.id) ? 'bg-green-50' : ''}`}>
+                              <td className="px-2 py-4 w-10">
+                                <input
+                                  type="checkbox"
+                                  checked={selectedFields.has(field.id)}
+                                  onChange={() => toggleFieldSelection(field.id)}
+                                  className="rounded border-gray-300 text-green-600 focus:ring-green-500"
+                                />
+                              </td>
                               {sortType === 'custom' && (
                                 <td className="px-2 py-4 whitespace-nowrap">
                                   <div className="flex items-center gap-1">