The purpose of this study was to examine the physical activity of school children during camping in a mountain environment. The subjects were 20 children (1^st-3^rd grade) who participated in the camp, which was carried out in a typical mountain village area of Tohoku district during five days in the summer vacation. Physical activity (steps and times engaged in light to moderate and vigorous intensities of physical activity) was measured by uniaxial accelerometer (Lifecorder^EX, Suzuken Co., Ltd.). The data were compared with those of previous studies on the physical activity of children during weekdays and weekends during school term using the same method. The number of steps during camp activity were similar to those on weekdays during school term, and greater than during holidays. Moreover, the times engaged in light to moderate activity such as standing activity, and fast-paced walking (2.0-6.0 Mets) during the camp were greater than those on weekdays at school and during holidays. These results suggest that children's experience of activities in a natural environment through the camp and interest in natural environments promote voluntary physical activity by children. Therefore, it is possible that such camp activity plays an important role in promoting the voluntary physical activity of children during the long or short vacation.