This paper examines the effects on mental task of changes in the intensity of physical activity. A treadmill-equipped instrument and perception tester were used to attain several levels of physical activity. In this paper, in order to determine the individual levels of physical activity of subjects, Borg-RPE scale, heart rate(HR) and respiratory quotient(RQ) were used. Also, an arithmetic addition test in whole-body activity on treadmill-equipped instrument as an indicator of mental task were performed. In the above experiments, the scores obtained in arithmetic addition test administered before and after physical activity at each intensity level used. Restricted within the limits of this paper, the results of these tests showed that the performance of mental task was Increased after physical activity.