PURPOSE: To acquire the optimum access time and to improve the processing speed in a simple constitution of a system clock controller by controlling the system clock frequency based on the decision output of the access time, the conflict, etc., of an external device received from an external device state deciding part.

CONSTITUTION: When a controller 11 gives the accesses to the external devices like an I/O 13, a memory element 14, etc., the information is supplied from the external device to receive an access. Then an external device state deciding part 15 decides the access time and the conflict state of the external device. Based on the decision output given from the part 15, a clock control part 16 is controlled and the system clock frequency is changed. The access time set by the controller 11 to the external device is optimized in response to the system clock. Thus an optimum access time is acquired in accordance with the external device in a simple constitution. Then the information processing speed is improved with a system clock controller.