PURPOSE: To prevent gas generated from a product from being accumulated between an indirect heating member and a heat treated member and to improve quality of the product by a method wherein the indirect heat generating member is provided with some air discharging holes.

CONSTITUTION: An indirect heat generating member 2 is positioned between a heat generating member 1 and a heat treated member 3 so as to cover the heat treated member 3. The indirect heat generating member 2 is formed with round holes 4 for facilitating discharging of gas generated from the product. The holes 4 are opened to be formed at a central part of an upper surface and both sides of it. With such an arrangement, the gas generated from the heat treated member 3 is discharged from a space defined by the indirect heat generating member 2 through the round holes 4 of the indirect heat generating member 2. Accordingly, a disadvantage such as a broken lead of the product may not be generated.