Mold 12 has an upper mold half 14 and a lower mold half 16. Pellet 34 obtained by cutting a resin composition into a predetermined size is placed into the mold 12. One pellet 34 is put on one recessed part 24. The pellet 34 has an incipient fluidization temperature T of 50.degree. C. or greater and 150.degree. C. or less. Weight of the pellet 34 is 1.4 g or greater and 2.0 g or less. Through clamping, the pellet 34 is compressed, and heated to allow the resin composition to flow thereby forming a preforming material. Difference (Tp-T) between the molding temperature Tp and the incipient fluidization temperature T is equal to or less than 70.degree. C. Percent Pp of change in the weight average molecular weight Mp of the preforming material to the weight average molecular weight M of the pellet 34 is equal to or greater than -30%. Cover is formed with this preforming material. Percent Pc of change in the weight average molecular weight Mc of the cover to the weight average molecular weight M of the pellet 34 is equal to or greater than -30%.