PURPOSE: To eliminate an omission of a read of data in a system wherein a computer writes data for update in a common memory at a period Tw and the other computer reads the data for update of the memory at a period TR longer than Tw.

CONSTITUTION: The update data write part of the common memory 3 consists of a data part 32 having plural (n) addresses, and the computer 1 shifts data in the data part 32 from the 1st-(n-1) addresses to the 2nd-(n)th addresses at each period Tw and writes the latest update data in the 1st address. Namely, update data of (n) times before the current point are stored in the data part 32 at all times. Simultaneously with the writing, the value of a counter part 31 is increased by one. The computer 2 knows the frequency A of writing of the update data written in the common memory 3 after its last read from the difference between a counter value at the time of the last read and the counter value at the time of the current read and reads the update data out as many times as the frequency, i.e., of the 1st (A)th addresses of the data part 32 to eliminate an omission of the read.