PURPOSE: To realize a highly accurate correction of time for a computer system having a hierarchical structure, by setting a circuit which receives an external clock to correct an internal clock in each computer.

CONSTITUTION: Equipped computers 40 and 60 contain the quartz oscillators 41 and 61 serving as internal clock generators and correcting circuits 42 and 62. Each correcting circuit consists of two OR gates 44 and 45 that deliver the reset signals and a counter 43 that counts the internal clocks. Then the clock given from an external clock generator 20, i.e., the time standard source of a host computer 10 is supplied to an NOR gate 44. Thus a counter 43 is reset to correct the internal clock. At the same time, the internal clock is corrected for other equipped computers. In such a way, a highly accurate correction is possible for the time.