The voluntary regulation of amount of exercise by teachers must be of great importance for the betterment of physical activity classes. The present study, therefore, was designed to obtain rational materials for establishing the method how to voluntarily regulate the amount of exercise and activity in physical education classes. The Subjective Intensity of Teachers Presumed (SIP), obtained from teachers with more than 5years experience and Rating of Perceived Exertion (RPE) of 5th and 6th grade boys were used in this study. Significant high correlation coefficients were found between the SIP and the RPE, and these relationships did not reflect individual difference among boys and teachers. The RPE of each boy well reflected the objective intensity of activity, particularly when ranked by their relative intensity evaluation. The satisfactory classes conducted for boys were to ; (a) allocate at least 12 minute for the main learning item with RPE of "not-light, but not-severe" ranking-level. It was found that, using the SIP based on the RPE of boys, teachers were able to regulate the amount of exercise at their satisfactory level in physical activity classes. The above-described results suggest that SIP of teachers may be applied to better the classes by their expert and voluntary regulation of the amount of physical activity classes.