increase strength as a result the lower case 22 b need not be formed to have a bottom across its entire lower side rather the lower case 22 b is formed to have a ring shaped edge 22 d just large enough to penetrate below part of the underside of the drawn portion 24 c the ring shaped edge 22 d defines a hole on its inner side through which the magnetic circuit board 24 a is exposed thus in this embodiment as described above the lower case 22 b need not be formed to have a bottom so as to cover the entire underside of the magnetic circuit board 24 a of coke 24 but is instead formed to have the edge 22 d so as to cover only the circumferential portion of the underside of the magnetic circuit board 24 a this eliminates the problem encountered in the prior art namely inadequate filling of molding resin caused by the resin not reaching the center of the bottom accordingly the thickness t 1 of the edge 22 d can be made less than 0 3 mm in this embodiment the thickness t 1 is set to 0 15 mm even if it is necessary to provide a thickness of 0 3 mm beneath the drawn portion 24 c in order to eliminate insufficient filling of resin at the time of molding and maintain a fixed strength in this embodiment it will suffice if the thickness t 2 from the leading edge of the drawn portion 24 c to the lower end face of the lower case 22 b is 0.