Abstract: A solenoid comprising a solenoid coil having a longitudinal hole at the center of a bobbin made of a non-magnetic material and a coil winding wound around the peripheral wall of the bobbin. A yoke comprising a peripheral side plate, an upper end plate and a lower end plate all for the covering the outer peripheral surface and both end surfaces of solenoid coil, a pole core disposed at one end of the longitudinal hole and a plunger fitted slidably into the other end of the longitudinal hole. The solenoid further comprises a protrusion member for forming an irregular magnetic field, formed integrally at the center of the yoke, and a recess for fitting the protrusion member from the end surface of the peripheral wall of the bobbin formed in the peripheral wall.