PURPOSE: To quicken troubleshooting and to attain the consecution of processing even when a component is deteriorated by using the firmware to decide which clock signal is to be selected among clock output signals of rectangular wave oscillators whose oscillating frequencies differ from each other.

CONSTITUTION: A switching control circuit 31 decides which oscillator among rectangular wave oscillators (OSC) 1-1-1-n is to be selected depending on an inputted clock mode signal 201. Clock selection signals 311-311-n are inputted respectively to AND circuits 32-1-32-n, and only an output of the AND circuits corresponding to the clock signal of the selected OSC goes to logical '1' and the other outputs are all logical '0'. Thus, any one of valid among clock signals 321-1-321-n being output signals of the AND circuits 32-1-32-n and a clock signal 331 is fed to an external circuit via an OR circuit 33. An OSC with a higher frequency is selected for the diagnosis at system raising to attain quick troubleshooting of deterioration or defect of a circuit component.