A sensor including a tube having a small-diameter portion loosely inserted into a tube insertion hole of an annular mounting fixture and a large-diameter portion having a diameter larger than an inside diameter of the tube insertion hole. A flow path is formed, so that when the large-diameter portion of the tube is locked on the rear end of the tube insertion hole, a rear end of a gap, defined between an inner circumferential surface of the tube insertion hole and an outer circumferential surface of the small-diameter portion of the tube, remains open. Further, the inner circumferential surface of the tube insertion hole and the outer circumferential surface of the small-diameter tubular portion of the tube are brazed together with a brazing filler which flows into the gap.