It was learned that in an insulation heating coil used for continuously heating a running steel sheet, the conventional insulated structure of the induction heating coil was selected focusing on the heat resistance and insulation ability of the insulation itself and cannot prevent a drop in insulation ability due to entry of fine metal particles (for example, zinc fumes) in the surroundings.Therefore, an insulated structure of induction heating coil preventing the entry of zinc fumes and other fine metal particles, not falling in strength even in a high temperature environment, and able to extend the service life of the induction coil is provided.Specifically, the surface of the induction heating coil is covered with a ceramic cloth made of alumina-silica ceramic long-fibers not containing boron and the surface of that is formed with a heat-resistant insulation layer made of a surface hardening ceramic material containing alumina or alumina-silica fine particles and alumina-silica ceramic short-fibers.