To prevent various lifestyle-caused diseases, mental health care in daily life is one of the most important issues in recent years. This paper proposes quantitative analysis method of mental stress using heart rate variability (HRV). Concretely speaking, we proposed the geometric figure analytical method of HRV in consideration for respiration sinus arrhythmia (RSA). Then mental stress is quantitative evaluated by analyzing the characteristic of its plots. Experiments were repeated 51 times on 3 healthy male volunteers. The proposed method was confirmed efficiently, because of the measurement time is about half and the measurement dispersion is decreased in comparison with the conventional analysis methods.