PURPOSE: To make a stopper arbitrarily adjustable to a desired temperature by dividing a knob part into a grip hood and a grip cap.

CONSTITUTION: The knob part is divided into the grip hood 4 and the grip cap 5. Thus, the position of the grip cap 5 in the axial direction is held unchanged independent on the amount of screwing of an adjusting nut 2. Further, the relative circumferential position of the grip hood 4 and the grip cap 5 is made arbitrarily changeable by providing to the grip hood 4 a slot 41 through which a ring segment 52 including a releasable stopper 51 penetrates. Thus, the axial position of the knob does not change in screwing of the adjusting nut 2, and the stopper 51 can be arbitrarily adjusted to a desired temperature.