When a power supply is cut off on updating programs (119, 120, 121), improper data remains since the power source to be supplied to the vehicle-mounted device (100) is unstable. The check for the improper data and restoration processing are required for a reactivation processing implemented by an update processing unit (122) so that the update is completed correctly, therefore, the user is waited. In contrast, the update processing also implemented by the update processing unit (122) for a time period during which the power source becomes unstable is interrupted, and implemented for the time period during which the power source voltage is stable. In consequence, the update processing is carried on steadily without making the user wait.