A seal member (60) has a dual-tube structure consisting of an outer tubular seal member (160a) and an inner tubular seal member (160b); the outer tubular seal member (160a) is welded, at its rear end portion, to the inner circumferential surface of a forward-end housing (131), thereby forming a housing-side joint W2; the inner tubular seal member (160b) is welded, at its forward end portion, to a forward end portion of the outer tubular seal member (160a), thereby forming an inter-tubular-seal-member joint W3; and the inner tubular seal member (160b) is welded, at its portion located rearward of the joint W3, to the outer circumferential surface of the heater (10), thereby forming a heater-side joint W1. An action of pressing the heater forward is effected on the basis of an axial dimension L1 between the joints W2 and W1 shorter than an axial dimension between the joints W2 and W3 and is, therefore, mitigated.