PURPOSE: To eliminate useless waiting time when a high-speed CPU between CPUs which are greatly different in processing speed acquires the right to exclusively access a common memory.

CONSTITUTION: The CPUs 1 and 2 when making requests to exclusively access the common memory 3 sets exclusive request flags F1 and F2 in exclusive request flag holding parts 6 and 7 and sets the exclusive identifiers of the opposite CPUs in an access arbitration word holding part 5. At this time, the low- speed CPU 1 sets both the high-order bytes 5a and low-order bytes 5b of the holding part 5, but the high-speed CPU 2 sets only the low-order bytes 5b. Then the identifier which is set later is left in the access arbitration word holding part 5. It is judged that the CPU 1 obtains the right to exclusively access the common memory from certain states (1)-(3). Namely, (1) the exclusive request flag F1 of the CPU 1 is not set, (2) the exclusive identifier of the CPU 2 is set in the low-order bytes 5b unless the state (1) is obtained, and (3) neither identifier is set in the high-order bytes 5a unless the state (2) is obtained.