diff --git a/Tests/PhotochkTests/SelectionTests.swift b/Tests/PhotochkTests/SelectionTests.swift
new file mode 100644
index 0000000..b4a4fe2
--- /dev/null
+++ b/Tests/PhotochkTests/SelectionTests.swift
@@ -0,0 +1,102 @@
+import Foundation
+import Testing
+
+@testable import Photochk
+
+@Suite("選択ルール")
+@MainActor
+struct SelectionTests {
+    private func asset(
+        _ name: String, width: Int, height: Int, size: Int64, days: Double
+    ) -> PhotoAsset {
+        PhotoAsset(
+            url: URL(fileURLWithPath: "/tmp/photochk/\(name)"),
+            fileSize: size,
+            modificationDate: Date(timeIntervalSince1970: days * 86_400),
+            captureDate: Date(timeIntervalSince1970: days * 86_400),
+            pixelWidth: width,
+            pixelHeight: height
+        )
+    }
+
+    private func makeGroup() -> SimilarityGroup {
+        let big = asset("big.jpg", width: 4000, height: 3000, size: 5_000_000, days: 10)
+        let mid = asset("mid.jpg", width: 2000, height: 1500, size: 2_000_000, days: 20)
+        let small = asset("small.jpg", width: 1000, height: 750, size: 500_000, days: 1)
+
+        return SimilarityGroup(
+            members: [
+                .init(asset: big, relation: .exact),
+                .init(asset: mid, relation: .similar(distance: 0.1)),
+                .init(asset: small, relation: .similar(distance: 0.2)),
+            ],
+            representativeID: big.url
+        )
+    }
+
+    private func makeModel() -> ScanViewModel {
+        let model = ScanViewModel()
+        model.setGroupsForTesting([makeGroup()])
+        return model
+    }
+
+    @Test("最後の1枚は選択できない")
+    func keepsAtLeastOne() {
+        let model = makeModel()
+        let group = model.groups[0]
+        let ids = group.members.map(\.id)
+
+        model.toggleMark(ids[0], in: group)
+        model.toggleMark(ids[1], in: group)
+        #expect(model.markedCount == 2)
+
+        // 3枚目を選ぶと全滅するので拒否される
+        #expect(model.canMark(ids[2], in: group) == false)
+        model.toggleMark(ids[2], in: group)
+        #expect(model.markedCount == 2)
+
+        // すでに選んでいるものは解除できる
+        #expect(model.canMark(ids[0], in: group) == true)
+    }
+
+    @Test("解像度が最も高い1枚を残す")
+    func keepHighestResolution() {
+        let model = makeModel()
+        model.applyKeepRule(.highestResolution)
+
+        let kept = model.groups[0].members.filter { !model.markedForDeletion.contains($0.id) }
+        #expect(kept.count == 1)
+        #expect(kept[0].asset.fileName == "big.jpg")
+    }
+
+    @Test("最も新しい1枚 / 最も古い1枚を残す")
+    func keepNewestAndOldest() {
+        let model = makeModel()
+
+        model.applyKeepRule(.newest)
+        var kept = model.groups[0].members.filter { !model.markedForDeletion.contains($0.id) }
+        #expect(kept.map(\.asset.fileName) == ["mid.jpg"])
+
+        model.applyKeepRule(.oldest)
+        kept = model.groups[0].members.filter { !model.markedForDeletion.contains($0.id) }
+        #expect(kept.map(\.asset.fileName) == ["small.jpg"])
+    }
+
+    @Test("選択解除ですべての印が消える")
+    func clearMarks() {
+        let model = makeModel()
+        model.applyKeepRule(.highestResolution)
+        #expect(model.markedCount == 2)
+
+        model.clearMarks()
+        #expect(model.markedCount == 0)
+        #expect(model.markedBytes == 0)
+    }
+
+    @Test("選択した容量の合計が集計される")
+    func markedBytes() {
+        let model = makeModel()
+        model.applyKeepRule(.highestResolution)
+        #expect(model.markedBytes == 2_500_000)
+    }
+}