To facilitate temperature control when a cam piece is to be heated.

A heating means 42 is inserted into an opening 26 in a cam piece 18, and a plurality of plates 43 deform uniformly to make contacting with the inside surface of the opening 26 uniformly over the whole circumference. Further, the plates 43 are put in pressure contact with the inner side face of the opening 26 by an elastic force generated by deformation of the plates 43. Even in the case the heating means 42 is inserted in the condition that its axis out of alignment from the center of the opening 26, the plates 43 deform appropriately along the inner side face of the opening 26 so that all of the plates 43 are in tight attachment to the inner side face. That is, the area of tight attachment to the inner side face is constant irrespective of the condition in which the heating means 42 is inserted in the opening 26, and the amount of heat conduction per unit time is stabilized at all times. Because the plates 43 are all heated to a temp. approx. the same as a plug heater 37, and it is possible to uniformly heat the inside circumferential surface of the opening 26 in a short time.