To double a battery capacity and enable various power controls by detecting a battery lock state interlocked with a lock means of a battery provided at the battery storage part by means of a sensor switch.

When the tip end parts of batteries 3 and 4 come into contact with the tip end of a lever 6, the lever 6 rotates clockwise against the energizing force of a coil spring 7, moves a lever 6, and a slider 10 of the lever 6 moves in the left direction. By means of the slider 10, a sensor switch 11 is constituted so that the switch can detect that either of batteries 3 and 4 is inserted into a battery insertion part 2 of a cabinet 1. When either of these batteries 3 and 4 is inserted into the battery insertion part 2, a tip end 12 of the lever 6 fits into a notch 13 of a battery 3 or 4, therefore, the lever 6 rotates with its center being a shaft 14 and is locked in the battery storage part 2.