Objects We have recently developed a new type of acceleration plethysmography (APG) machine and software. This APG system functions to indicate heart rate variability, by using the coefficient of the variation of the a-a intervals (CVaa%). We tried to evaluate the validity of the APG system to estimate heart rate variability, instead of using ECG. Methods We simultaneously recorded an ECG and an APG by using a 2-channel APG system. We examined the difference between the a-a intervals (Taa) of an APG and the R-R interval of an ECG. Next, APG waveforms of 121 healthy people and 26 diabetics were recorded, and the mean CVaa% in each age category for healthy people and diabetics were calculated. Results 1) All differences between the Taa and the R-R interval were within 10 msec., and the multiple correlation coefficient was 0.999175. 2) The mean CVaa% was 0.2 to 1.5% bigger than the mean coefficient of the variation of the R-R intervals (CVRR%) in all age categories. 3) The mean CVaa% decreased by age. 4) In ages 40-60, mean CVaa% of diabetics was significantly lower than that of healthy people. Conclusion The APG system is suitable to estimate heart rate variability. And, when we want to evaluate heart rate variability by CVaa% of an APG in stead of CVRR% of an ECG, a standard value of CVaa% in each age is necessary. The APG system has some possibilities for explanation of a mechanism of autonomic-nerve-related diseases.