To extend the lifetime of a battery by controlling supply of power from the battery to a main part and stop of power to the main part.

A main part 10 is connected to a battery E via a first relay switch SW1. The main part 10 has a microcomputer 11 for controlling the operation of a second relay switch SW2 through the operation of a transistor Q1. A switch P is closed when sensing a change in acceleration. A self-holding circuit comprising a relay switch SW2 and a first relay having a relay coil RY1 and the relay switch SW1 continues turning on the first relay switch SW1 when the switch P is closed. When the switch P is turned off, the first relay switch SW1 is turned off independently of the operation of the self-holding circuit by turning off the second relay switch SW2 to stop supply of the power from the battery E to the main part 10.