PURPOSE: To prevent a hot plate from being contaminated owing to adhesion of resin and to maintain excellent radiation characteristics by covering the hot plate, on which surface treatment being apt to generate far infrared rays is applied, with a cover.

CONSTITUTION: A hot plate 1 made of stainless (SUS 630) is covered with a cover 2 made of aluminum in a thickness of 2mm to cover a hot plate 1 surface. Ceramic coating (flame spraying of 46% CR2O3 and 26% Fe2O3) with a thickness of 80-100μm is applied on the surface of the hot plate 1. The cover 2 is formed such that electrolytic oxidation treatment is applied on both surfaces. When the cover 2 is contaminated and radiation characteristics are deteriorated, the cover is replaced by a new cover to maintain initial radiation characteristics.