A fixing plate for the thermal insulation material is attached to the combustion plate flange part. Provided that a direction leaving away from the air-fuel mixture ejection part of the combustion plate is defined as an outward side direction, there are disposed: on the side edge in the outward side direction of the fixing plate, bent edge parts, which come into contact with the side edge in the outward side direction of the thermal insulation materials; and a plurality of tongue-shaped pressing claws which are elongated from the bent edge part so as to press and prevent the thermal insulation materials from getting released away from the fixing plate.