PURPOSE: To simplify constitution and reduce dimension and economize electric power by installing a shape memory alloy member which attaches a heat radiation part onto a heating part against the urging power of an urging means by a contraction force at the drive of the heating part.

CONSTITUTION: When a gas decomposing part 11 and a heating part 12 are driven, the generated heat in the heating part 12 increases, and the temperature rises. Then, in a heat radiation shielding mechanism 16, a driving part 17a is shifted in the direction of arrow A and set close to the heating part 12 by the contraction of a shape memory alloy member 17. In this case, the shape memory alloy member 17 urges the driven part 15b of a heat radiating member 15 against the springy force of a spring member 19 by the driving part 17a, and the driving part 15a is attached onto the heating part 12. Therefore, the generated heat in the heating part 12 is irradiated outside through the heating member 15 and heat radiating coating part 15a, and a heat control is carried out. Therefore, the need of installing a heat insulating heater is obviated, and the dimension can be reduced, and the economization of electric power is contrived.