A pair of bosses are formed in the vicinity of a cut out portion of a lower casing from which a lead circuit section, which is integrally formed with a switch circuit section a flexible printed circuitry is led out. Three engagement holes are arranged in a longitudinal direction of the lead circuit section with predetermined intervals at both end portions of a widthwise direction thereof. The lead circuit section is folded such that all the respective three engagement holes are fitted with the respective bosses while providing surplus length portion in the form of S-shape, and then led out from the cut out portion. The upper casing having a pair of hold-down members are combined with the lower casing such that the hold-down members are engaged with the respective bosses to constitute a casing which serves as a switch unit.