diff --git a/chainer/functions/activation/leaky_relu.py b/chainer/functions/activation/leaky_relu.py
index 81282cfd4ecb..86a043a0329f 100644
--- a/chainer/functions/activation/leaky_relu.py
+++ b/chainer/functions/activation/leaky_relu.py
@@ -65,6 +65,7 @@ def __init__(self, x, y, slope):
 
     def forward_cpu(self, inputs):
         gy, = inputs
+        gy = gy.copy()
         if self.slope >= 0:
             gy[self.y < 0] *= self.slope
         else: