To enable sure mounting of a metal contact piece even when the space for soldering is small.

A circuit board 30 ahs a pattern surface 30a on the front side thereof, a mounting surface 30b on the rear side, and openings 30d, 30e and a notch 30f near the lower end 30c. A power supply battery 32 is connected to the circuit board 30 via the metal contact pieces 37, 38. The metal contact piece 37 consists of a contact piece body 42 and a connection part 43. An insert piece 4 is provided on the upper end of the connection part 43, insert pieces 45, 46 are provided on both side ends thereof, and a slip-off preventing part 47 is provided near the lower end thereof. The slip-off preventing part 47 projects in the direction orthogonal to the insert pieces 44, 45. The insert pieces 44, 45 are inserted through the openings 30d, 30e, and the notch 30f from the side of the pattern surface 30a, and the slip-off preventing part 47 is made to go round into the side of the mounting surface 30b to mount the metal contact piece 37. The slip-off preventing part 47 abuts the circuit board 30 to prevent the separation of the metal contact piece 37.