To obtain a heating method by which a cylindrical metal coil can uniformly be heated in high yield and in a short time.

The cylindrical metal coil is manufactured by winding so as to electrically insulate the surface of a hoop metal and leader metals are connected with the inner periphery and the outer periphery of the cylindrical metal coil, and the leader metals are wound with the electrically insulative heat insulation material to form the coil, and an iron core is passed through the inner part of the coil and also, an iron core is arranged at the outer part of the cylindrical metal coil so as to form a magnetic circuit with the above iron core, and a primary coil is wound on the iron core arranged outside the cylindrical metal coil, and a primary electric source is connected. Whereas, the outside and the inside of the cylindrical metal coil connected with the leader metal, forms a secondary circuit by short-circuiting with an electrode connected with electric-conductive member. By this method, a transformer is formed and the current is made to flow into the coil formed of the hoop metal by applying the voltage to the primary coil to heat the cylindrical metal coil.