The safety of cardiorespiratory function is an accentuated factor in exercise prescription for improvement of physical fitness. When regulating exercise intensity using a self-selected exercise intensity (SSEI) method, it is important to not only consider responses in cardiorespiratory function but also the safety of exercise intensity. In this study, therefore, we compared changes in cardiorespiratory function during exercise at anaerobic threshold (AT) and by the SSEI method. In addition, we attempted to acquire new information regarding the safety of the SSEI method. Group 1 consisted of 28 healthy men, aged 25.5 ± 2.7 years, who completed two 20-min exercise trials. During the first trial, participants performed cycling for 20 min by the SSEI method (SS-trial). The second trial consisted of 5 min of fixed-load cycling at 70% (high intensity) of Vo_2max (SSFL_70%) followed by 15 min of cycling at SSEI. Group 2 consisted of 10 healthy men (25.0 ± 1.8 years) who completed two 20-min exercise trials at AT (AT-trial) and Above AT (Ab_AT-trial). Oxygen uptake during the SS-trial increased to the same level as that during the AT-trial. However, it was noteworthy that the exercise intensity selected by each subject was maintained at approximately 65%Vo_2max during SSFL_70%. These data suggest that a constant-load exercise relative to Ab_AT level can be sustained for a prolonged period of time without inducing a marked risk for the cardiorespiratory system.