To make it possible to stop or start a computer only by simply depressing a button.

When a stop/start key 66 on a keyboard 14 is depressed, a system management interruption generation logic 108 generates a keyboard interruption IRQ in a status register 112 on the basis of an interruption from a keyboard controller 100. Thereby, the register 112 receives an event for instructing that the computer is to be turned to be in a low power mode or a normal power mode. When power is normally supplied to the computer system, the keyboard interruption IRQ is not led out to the register 112. When the power of the computer system is switched off, a keyboard interruption IRQ1 is led out and the computer system is started by the action of the stop/start key.