PURPOSE: To prevent the intrusion of dust from an opening of a housing with a slide plate, and to form an operation unit large to improve the operability.

CONSTITUTION: A slide plate 54 is arranged inside of a housing 20 so as to cover an opening 20a of the housing 20. A knob 51 formed with an operation unit 51a and a base 52 connected to a lower part of the knob 51 are fitted to the slide plate 54 freely to be slid so as to pinch the slide plate 54. A slider 53 is fitted to the lower surface of the base 52. A wiring pattern 83a of a printed substrate 83 is shortened and opened by the slider 53 in response to the movement of the knob 51.