The Japanese marten (Martes melampus) invades houses through holes and causes excrement problems. These holes should be covered to prevent the marten's intrusion into houses. However, there is no information about what size of hole they can squeeze through. Therefore, we investigated the sizes of the holes that they could pass through using three reared individuals in summer and winter. We put a bait in an experimental box with a replaceable front panel, which had a square or round hole. The minimum passable sizes were recorded in summer. For a square hole the minimum size was 5.0 cm for each side, and for a round hole the minimum sized hole had a diameter of 5.5 cm. It is necessary to cover holes of these sizes or more to prevent their house invasion. The body weight and the hole shape had no significant effect on the minimum passable area. On the other hand, the season had a significant effect on the minimum passable areas. The shoulder seems to be the limiting factor for whether the martens can pass through the holes. Therefore, the seasonal change in the body size around the shoulder may affect the minimum passable area of holes.