The Hydrographic Department of Japan has been performing a Satellite Laser Ranging observation at Simosato, central Japan, since 1982. It has been reported that the range data at Simosato has constant range biases of several centimeters based on the quality check analyses, e.g. at CSR, University of Texas. The cause of the bias may be an error (1) in the correction of the system internal delay, and/or (2) in the time detection of the return signal due to the threshold strategy; the difference in amplitudes of return pulses between from the calibration survey and from the satellite tracking may cause some biases. A closer look at the time series of estimated range biases of Simosato data for LAGEOS and Ajisai on our own analyses shows that the bias for Ajisai is a little smaller than that for LAGEOS. In view of the fact that the return signal of Ajisai is generally stronger than that of Lageos, this is consistent with the latter cause above. A simple ranging experiment at Simosato also suggests that the difference in signal strength causes the bias of several centimeter level.