The aim of this study was to determine whether the objective activity level of children with attention deficit/hyperactivity disorder (AD/HD) is associated with situational factors in elementary school. A total of 16 children with combined-type AD/HD (13 boys, 3 girls) and 20 controls matched for age and gender participated in the study. All the participants wore the objective measurement of activity level for 1 week while attending elementary school. The average activity levels were calculated for the morning and afternoon classes. The classes were categorized into four types: (i) in-seat classes, in which the participants were expected to learn quietly while sitting in their own seats; (ii) not-in-seat classes, in which the participants were not expected to sit in their seats; (iii) physical education classes; and (iv) lunch/recess periods. During the afternoon in-seat classes, the children with AD/HD were significantly more active than the controls. However, no significant differences based on the group (children with AD/HD or controls) were observed during the other morning or afternoon classes. These findings indicate that in naturalistic settings where the effects of situational demands involving inhibition and the effect of fatigue overlapped, the children with AD/HD exhibited an activity level that was different from that exhibited by the controls. Differences in the activity levels of children with AD/HD and normal controls were identified using an objective measurement of activity level when two situational factors overlapped.