In order for a conduction path to have a reduced number of sliding portions for conduction, without increase in inductance nor resistance, thereby permitting an enhanced accuracy of inspection, a pair of plungers (3, 4) biased in opposite directions by a coil spring (2), to be electrically connected to a wiring plate (10), have electrical connections in which, in a tubular portion (15) as a tight wound spiral portion (15a) fixed on one plunger (4) to allow linear flow of electrical signal, the other plunger (3) is brought into slidable contact.