A sampling voltage is periodically applied between contacts of a switch of a device to determine a status of the switch. When a CPU is in a sleep state and the status of the switch is determined closed, the CPU is activated. The sampling voltage when the CPU is in the sleep state is adjusted lower than the one when the CPU is in the wakeup state. This reduces power consumption for the switch status determination. The sampling voltage adjusted at the lower level periodically adjusted to a higher level to reduce errors in the determination due to insulation films on the contacts.