This study was conducted to develop an open-circuit metabolic apparatus equipped with a treadmill for obese-diabetic model rats, which can not be examined using ready-made apparatuses as the chamber volume is insufficient, and to examine the effects of 10-week exercise training with three different intensities on aerobic work capacity, body composition, and biochemical properties in skeletal muscle. Flow rates through the chamber were established by measuring chamber-clearance time. Thirty-six male OLETF rats, 20 weeks in age, were divided into three exercise groups and a sedentary control. The study was conducted with a 10-week, 60 min/day, 5 days/week exercise training regimen using a motor-driven treadmill. VO_2max, oxygen uptake accumulated (ΣVO_2) during an exhaustive exercise, and exhaustion time (ET) were measured pre- and post-training using the apparatus developed in this study. ΣVO_2 and ET increased depending on the increase in exercise intensity. Visceral and subcutaneous fat weights decreased, and additional weight of muscles per body weight (%ΣMuscle) increased, depending on the increase in exercise intensity after the training. ΣVO_2 was significantly correlated to %ΣMuscle (r=0.490, p<0.01), and the correlation coefficient between ΣVO_2 and CS activity (r=0.378, p<0.05), measured as an indicator of aerobic work capacity in skeletal muscle, was somewhat lower. These results suggest that ΣVO_2, rather than VO_2max, is a suitable indicator of aerobic work capacity in obese-diabetic model OLETF rats, which are changeable in body weight, and that CS activity in skeletal muscle is not a representative indicator of aerobic work capacity in the whole body. It was, moreover, suggested that exercise training intensity had a specific influence on visceral and subcutaneous fat weights, and an obscure effect on CS and 3-HAD activities measured as indicators of oxidative and fatty metabolism in skeletal muscle.