There are many studies with respect to the age-related change of the characteristics of beat-to-beat heart rate variability (HRV), reflected by cardiac autonomic control, especially focusing on adulthood (i.e., aging related to the incidence of metabolic syndrome) in Japanese individuals. However, it is not still clear how basic control matures during childhood. This study was, therefore, designed to explore the HRV characteristics of pre- and post-adolescent Japanese, in a cross-sectional manner. Resting HRV data was recorded in a relaxing supine position from 136 healthy individuals between 8 and 20 years (48 boys between 8 and 14 years; 88 girls between 8 and 20 years) who were instructed to breathe periodically (0.25 Hz). Frequency-domain analysis (i.e., the spectral analysis based on an autoregressive model) of short-term, stationary R-R intervals was performed to evaluate the low- (LF; below 0.15 Hz) and high- (HF; 0.15-0.40 Hz) frequency powers. The HF to total power represents the vagal control of heart rate (PNS indicator), and the ratio of LF to HF (LF/HF) is considered to relate to the sympathetic modulations (SNS indicator). Both PNS and SNS indices had substantially no effect from age and/or gender in the range between 8 and 20 years. In conclusion, the control of the cardiac autonomic nervous system in Japanese seems already to be compatible with that in adulthood before approximately 10 years. In other word, the cardiac autonomic modulation would presumably be maturated before the age of approximately 7-8 years, though further research is awaited.