PURPOSE: To prevent varnish from flowing out and to uniformalize impregnation of varnish by a method wherein the varnish impregnated between a case body and a magnetic assembly mounted in the case body is formed of photo setting varnish.

CONSTITUTION: A magnetic assembly 1 comprising a core body having a coil 3 housed therein is fitted in a case body 13 substantially U-shaped. Next, photo- setting varnish 19 is applied on the end face of the core body, so that the varnish is impregnated into the joined part between the case body 13 and the core body. Thereafter, the varnish is hardened by irradiation of photo rays and then dried under heating. Because of using photo-setting varnish, it becomes possible to uniformly impregnate the varnish between the core body and the case while preventing the varnish from flowing out, and also to avoid adhering of the varnish onto terminal parts.