PURPOSE: To equally allocate priority while maintaining the consistency of a system in respect to respective processing request signals outputted from plural devices.

CONSTITUTION: A processing request signal discriminating circuit 5 discriminates whether the processing request signal outputted from a peripheral device (a) is single or plural and according to this discriminated result, a priority change control circuit 6 inhibits the change of the priority held in a priority register 2 to a priority change circuit 3 when there is the single processing request signal, and the circuit 6 permits the change of the priority held in the priority register 2 to the priority change circuit 3 when there are the plural processing request signals.