To precisely keep the clock synchronization of a system even when a time signal from a GPS communication satellite can not be received by correcting sec information of a real time clock(RTC) when the value of a PPS signal counter(PSC) and sec information of the RTC are not coincident with each other.

A PPS signal is generated even when a GPS receiving part 106 can not receive a time signal because of some receiving fault, etc. Then, even in this case, a PPS receiving part 107 receives the PPS signal and provides a PPS clock to a CPU 101. On the other hand, a local clock generating part 102 plays the role of generating the RTC and when timing slip is generated, it is corrected based on the PPS clock under control of the CPU 101. Namely, the CPU 101 monitors whether the PPS clock and the RTC are coincident and when both of the signals are not coincident with each other, corrects the timing slip.