Conventional behavioral tests for rodents are designed to be used under simplified artificial conditions for a short observation period. To analyze "usual behaviors" under less artificial conditions, we here developed a novel assay system that can automatically analyze behaviors of mice on the individual basis under group-housed conditions, and applied it to examine influences of various growing environments on behaviors. We first developed an ID marking technique and a software that can identify an individual mouse from camera images and record the location of each mouse as an XY coordinate. Then, C57BL/6J mice were divided into two groups, a group-reared (GR) group and an individually-reared (IR) group at weaning, followed by further rearing until adulthood. For the behavioral test, four male mice, which were strangers to each other, were introduced into a test cage and observed for 3 days. In the experiments, three combinations of mice, i.e., 4 mice from GR only (GR-4), 4 mice from IR only (IR-4), and 2 mice each from GR and IR (GR-2+IR-2), were used. Since mice have a circadian rhythm, activities averaged per a prolonged time, i.e., 60 min, were synchronized not only within a cage but also between separate cages. In contrast, average activities analyzed per a short time, i.e., 10 min, were synchronized in a group of mice kept within a cage only. This result indicates that an interior factor, e.g. social interaction, as well as an exterior factor, e.g. light-dark cycle, determines each mouse's activities. Mice have a tendency to huddle together with time in a home cage. IR mice in IR-4 needed more time until huddle than GR mice in GR-4 in a test cage. In GR-2+IR-2, the huddling behavior occurred first in GR mice, and then IR mice joined the group. Furthermore, a number of access of IR mice to GR mice was greater than that of the opposite direction. In conclusion, our newly developed assay system can be used to characterize not only behaviors of an individual mouse but also social interactions under group-housed conditions.