PURPOSE: To enable to obtain the right of usage in good order, according to the usage request with a simple constitution, by obtaining the right of usage of common bus through stopping the oscillation of loop oscillated in a give period with the usage request of each control unit.

CONSTITUTION: The control units 11 ... 1n are commonly connected to the common bus 20, and the common memory 30 can be accessed from either of the units 11 ... 1n. Then, the circuit stopping the loop oscillation by holding the output level constant is inserted serially to the loop oscillated in a given period when the units 11 ... 1n make usage request, allowing the control unit stopping the loop oscillation with the usage request of itself to obtain the right of usage for the bus 20. Thus, the right of usage can be obtained in good order according to the usage request with a simple constitution.