Ungulate populations such as deer and wild boars have been expanding in range and increasing in number throughout many areas of the world. Regulation of these overabundant populations is urgently needed. For an effective culling program addressing these overabundant populations, it is essential to have information on the activity pattern of animals. To determine the factors affecting seasonality of daily activity pattern in sika deer, we studied deer activity through camera traps on Nakanoshima Island, Lake Toya in Hokkaido, Japan, where human disturbance is low. We estimated activity pattern during four time periods (dawn, dusk, day, and night), and evaluated the effects of weather conditions on activity pattern (camera trap rate) for each period. Deer activity at dawn, dusk and night showed clear seasonal patterns, with peaks in September, while the activity pattern during the day was constant in all seasons. The activity at dawn and dusk tended to be higher than that at day during July–October and July–November, respectively. Although the activity of sika deer may be influenced by weather condition, there was no seasonal consistency. Our study reveals that human disturbance might decrease diurnal activity and increase nocturnal activity.