PURPOSE:To enable the automatic assembly by locating an iron core on the bottom surface of a box base of which upper surface is opened so that the opening directs the upper, and inserting a coil into a side piece of the iron core for fitting, and placing a permanent magnet on a central piece of the iron core, and bridging a contact spring over a place stand for arrangement, and engaging an oscillation supporting point with a projecting stand to be fitted freely to oscillate. CONSTITUTION:An iron core 1 is located on the bottom surface of a box base 11 of which upper surface is open so that the opening directs the upper, and a coil 2 wound around of a coil bobbin is inserted to a side piece of the iron core 1 for fitting, and a permanent magnet 4 is placed on a central piece of the iron core 1, and a contact spring 5 is bridged over placing stands respectively formed in both sides of the base 11 for arrangement, and oscillation supporting points 8 are engaged with projecting stands 17 respectively formed in both sides of the base 11 to be fitted freely to oscillate, and all parts are built-in the base by piling. Automatic assembly is thereby possible.