To facilitate making the physical fitness programs for obesity control, we investigated the effects of obesity on the ventilatory response during exercise and exercise tolerance. One hundred eighteen adults were divided into a normal group and fatty group by percentage of body fat. Each subject performed a submaximal exercise test under the gas-exchange measurement. The fatty group was inferior to the normal group in exercise tolerance. The respiratory mode, during exercise, in the fatty group was characterized by higher respiratory rate and less tidal volume compared with the normal group. This mode may result in increased dead space and therefore unavailable ventilation capacity. These data suggest that the characteristic respiratory mode in the fatty group was a factor in the lower exercise tolerance.