External factors that affect seasonal coat color changes in the Japanese marten (Martes melampus) were examined under experimental conditions. In martens, captured in Kagoshima Prefecture and exposed to a photoperiod of 10L: 14D with light intensity of 20–70 lux, molting rhythms did not synchronize with each other over six years, regardless of ambient temperature fluctuations. The cycle length from the start of one blackish color change to the next was approximately 14 months. This may be caused by a modifiable endogenous circannual rhythm that may be reset by external photoperiods. For the male kept at 14L: 10D with light intensity of 600 lux, duration of activity was 13 h, and his fur remained blackish without change for three years. The fur of female, kept under the same conditions, changed to yellowish when the duration of activity changed to 9 or 10 h, and subsequently remained brackish without molting when the duration changed to 13 h. These results show that length of time exposed to light per day is an important factor that affects coat color changes. The camera trapping survey revealed that martens with brackish coat color in winter may inhabit Kagoshima Prefecture without molting although they were rare.