To ensure an improved rustproof property of magnetic metal material by spraying it on the external surface of inside pots from the bottom to the corner and side so that it makes no boundary steps with the inside pot body, and ensure higher heating efficiency by transferring the heat the magnetic metal material produces to the inside pots without any external loss.

A body 1a of an inside pot 1 is made of such a nonmagnetic metal as aluminum. The inside pot body 1a is provided over its external surface with a sprayed layer of such a magnetic metal material 1b as iron having thicknesses gradually reducing as reaching the side via the corner from the bottom made thickest which faces an electromagnetic induction heating coil. Over the magnetic metal material 1b, a rustproof layer 1d of ceramic material is further laid to extend beyond it.