PURPOSE: To obtain an insulating protective film possessed of a precise and required thickness by a method wherein a process is repeatedly executed, where fine particles are made to adhere to the surface of an element through an electrophoresis method, which are heated, melted, and solidified to form a glass coating.

CONSTITUTION: An insulating protective film 6' is formed by heating, melting, and solidifying glass fine particles adhered through a first electrophoresis method. An insulating protective film 6" is formed by heating, melting, and solidifying glass fine particles adhered through a second or an electrophoresis method performed thereafter. The protective film 6" is completely formed through a second and a succeeding heating, melting process on a rugged part 7 of an element where the glass protecting film 6, has been insufficiently formed or not been formed. By these processes, a complete insulating protective film having a required thickness can be obtained.