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. <EOS>