PURPOSE: To synchronize the times of the system devices which are connected to one network without using any special external device by finding a transfer time and the time difference from a reference system device according to a specific time.

CONSTITUTION: A system device 2 whose time is to be synchronized transfers the time T2 when the time of the reference system device 1 is received, a time T1 transferred from the device 1, and a time T3 just before the time is to be transferred to the device 1, i.e. three times T1 - T3 to the device 1. The device 1 finds the transfer time and the time difference from the device 1 from a calculation expressing based upon the time T4 when the three times T1 - T3 are received and the three times T1 - T3, and transfers it to the device 2 whose time is to be synchronized, so that the time difference is set. Consequently, when there are plural system devices in the same network, synchronous setting becomes possible on the basis of the time of the reference system device and the respective system devices without any consideration of the processing time in a computer.