diff --git a/TIASshot.Tests/GainCalcCalcGainValueTests.cs b/TIASshot.Tests/GainCalcCalcGainValueTests.cs
index 2a331ca..1aa3035 100644
--- a/TIASshot.Tests/GainCalcCalcGainValueTests.cs
+++ b/TIASshot.Tests/GainCalcCalcGainValueTests.cs
@@ -115,6 +115,18 @@
             Assert.That(result, Is.EqualTo(133));
         }
 
+        /// <summary>
+        /// CalcGainValue_現在値0で比率がInfinity_範囲内に収まる:
+        /// ホワイトバランスが下限 0 に張り付いた状態で白パッチの計測値が 0 になると
+        /// 0 * Infinity = NaN となり範囲判定をすり抜ける．未定義値（int.MinValue）を
+        /// 設定せず，必ず [rangeMin, rangeMax] に収まること
+        /// </summary>
+        [Test]
+        public void CalcGainValue_現在値0で比率がInfinity_範囲内に収まる() {
+            int result = GainCalc.CalcGainValue(0, float.PositiveInfinity, 0, 255);
+            Assert.That(result, Is.InRange(0, 255));
+        }
+
         // ---------------------------------------------------------------
         // CalcRatio との結合（実際の校正経路の再現）
         // ---------------------------------------------------------------