A pressing member is provided so as to be capable of moving to an ON position and an OFF position. With the pressing member in the OFF position, a first contact point and a second contact point assumea non-contact state. With the pressing member in the ON position, if the pressing member presses against a contact piece, the first contact point and the second contact point assume a contact state.An actuator moves the pressing member from the OFF position and, passing through an overshoot position beyond the ON position, to the ON position. The contact piece comprises a main body and a low rigidity portion which has a lower rigidity than that of the main body. The pressing member presses against the low rigidity portion.