PURPOSE: To eliminate a need of wait for the end of invalidating processing by advancing a process while giving a condition of a computer where read of new data is guaranteed.

CONSTITUTION: When the process issues a request (a) of file change in one computer, a set R(a)1 constituted by the computer where the file is changed and computers which do not have the changed file is formed. When invalidation is executed in all computers of the network, which have the old file, by an invalidating request, all computers are placed in the set R(a)1. The system is so constituted that R(a)≥R(b) is always true when the process which issues the request (a) or another process of a computer in the set R(a)1 issues another write request (b) of file change after reading data rewritten by the request (a). Consequently, the process which issues the write request is advanced without waiting for invalidation of all copies.