This study investigates and compares the exercise intensity of the three exercise tasks-play activity, practce, and time trial-in swimming lessons for the subjects from infants to college students. The resuls has shown the exercise intensity of the three exercise tasks. However, they are supposed to have enough quantity of motion for their growth and development, adapt themselves to water, and acquire the basic skills of swimming by way of carefully arranged palys in the water which are based on the system of swimming skills. 2. Aerobic power of elementary school children was expected to improve by the three exercise tasks. Thus, it is important to modify the lessons to include the three exercise tasks for the improvement of aerobic power. 2. Aerobic and anaerobic power of middle school students was expected to improve by play activities and practice, and yet anaerobic power was not improved by time trial because of too long rime to rest. Thus, it is desirable to carry out time trial with short rest. 4. Practive for high school students was so low that their aerobic power was not improved. Thus, it is desirable to carry out interval training with long swimming distance per time and short rest as basic practice. Futhermore it is necessary to extend the swimming distance per time in time trial to 50 meter so as to activate the energy system of lactic acid. 5. Aerobic and anaerobic power of college students was expected to improve by practice and time tral. However, further study on the methods is needed for the improvement of aerobic and anaerobic power by play activities.