The inflow port 3a is provided in the base plate 3, and the tubular insulating member 10 with the electromagnetic induction coil 25 wound therearound is integrally assembled onto the base plate 3 through the outflow pipe 35. Therefore, by a simple operation of detaching the bottomed tubular container 2, which is put such as to cover the tubular insulating member 10 integrally assembled onto the base plate 3, from the base plate 3 by unfastening the bolts 4 and nuts 5, the tubular insulating member 10 integrally assembled onto the base plate 3 is exposed to the exterior. Consequently, maintenance of the electromagnetic induction coil 25 and the heating magnetic body 20 and the like can be carried out easily.

The ceramic-made tubular insulating member 10 with the heating magnetic body 20 accommodated therein and with the electromagnetic induction coil 25 wound therearound is unitized by attaching the metallic cylindrical end members 11 and 12 respectively to both ends thereof. Therefore, by detaching the thus unitized assembly from the outflow pipe 35, which penetrates and is secured to the base plate 3, and by unfastening the bolts 37 and the nuts 38, the unit as a whole can also be replaced easily.