A solenoid comprising: a cylindrical case having an opening; a permanent magnet and a coil both built into the cylindrical case, wherein the permanent magnet and the coil are arranged separated in a length axial direction in the case so that the permanent magnet is located nearer to the opening than the coil is; a first ring member having a center hole and an outer periphery which is arranged in the case adjacently to the permanent magnet on the far side of the permanent magnet from the opening; a movable iron core which is inserted in the coil so as to be arranged on a far side of the first ring member from the opening; a metallic coil cover having a flange on its end facing the first ring member which cover is provided between the movable iron core and the coil; and a second ring member which is arranged on an opposite side of the coil to the flange and which is fixed to the case, wherein the coil cover and the second ring member together with the case completely cover the coil, characterized in that the first ring member is arranged to form a predetermined distance (d) in the radial direction of the first ring member between the outer periphery of the first ring member and an inner wall of the case, in that a first magnetic path is formed through the first ring member, the movable iron core, the coil cover, the second ring member, the case and the flange by the permanent magnet during non-energization of the coil, a second magnetic path is formed through the coil cover, the second ring member, the case and the flange by the coil during energization of the coil, a third magnetic path is formed through the first ring member, the space forming said distance and the case by the permanent magnet during energization of the coil, and in that a diameter of the iron core is larger than a diameter of the center hole of the first ring member, the diameters being taken perpendicular to said length axial direction. a cylindrical case having an opening; a permanent magnet and a coil both built into the cylindrical case, wherein the permanent magnet and the coil are arranged separated in a length axial direction in the case so that the permanent magnet is located nearer to the opening than the coil is; a first ring member having a center hole and an outer periphery which is arranged in the case adjacently to the permanent magnet on the far side of the permanent magnet from the opening; a movable iron core which is inserted in the coil so as to be arranged on a far side of the first ring member from the opening; a metallic coil cover having a flange on its end facing the first ring member which cover is provided between the movable iron core and the coil; and a second ring member which is arranged on an opposite side of the coil to the flange and which is fixed to the case, wherein the coil cover and the second ring member together with the case completely cover the coil, characterized in that the first ring member is arranged to form a predetermined distance (d) in the radial direction of the first ring member between the outer periphery of the first ring member and an inner wall of the case, in that a first magnetic path is formed through the first ring member, the movable iron core, the coil cover, the second ring member, the case and the flange by the permanent magnet during non-energization of the coil, a second magnetic path is formed through the coil cover, the second ring member, the case and the flange by the coil during energization of the coil, a third magnetic path is formed through the first ring member, the space forming said distance and the case by the permanent magnet during energization of the coil, and in that a diameter of the iron core is larger than a diameter of the center hole of the first ring member, the diameters being taken perpendicular to said length axial direction.