A new physiological index (μPA) is proposed to represent the autonomic nervous system (ANS) function. The index μPA is defined as the natural logarithm of the ratio between two different frequency components of the pulsatile amplitude of the photoplethysmogram (PPG) signal. The discrimination ability and the reproducibility of μPA have been compared with other traditional ANS indices. In the experiment, the electrocardiogram, the PPG and continuous blood pressure were measured in 59 healthy young subjects (age 25.7 ± 6.3) and 86 healthy elderly subjects (age 70.2 ± 4.1) at rest. The discrimination ability and the reproducibility were evaluated by Cohen's d between young and elderly groups and by the interclass correlation coefficient, respectively. The results showed that the elderly subjects were significantly (p<0.001) lower than young subjects in μPA and a few traditional indices introduced to be compared with μPA. Therefore, it suggests that μPA is associated with the decrease in the ANS function accompanied by aging. Moreover, it showed that the discrimination ability and the reproducibility of the proposed index are comparable or larger than those of traditional indices. The proposed index based on the PPG signal will be applied to tele-healthcare systems for monitoring people's health in daily life in combination with the ratio of the standard deviation of the R-R intervals to their average value (CVRR).