PURPOSE: To make the wall thickness of a molded article uniform by keeping slipplage of an elastic sheet and a cavity surface excellent.

CONSTITUTION: From the part of an elastic sheet 10 coming into contact with parison 12 toward the cavity surface 8 side, a viscous material 3 is provided beforehand so that it can be restored to an initial state, and a resin molded article is obtained by blow molding the parison 12 in this state. By the action of the viscous material 3, the elastic sheet is made to slip in relation to the cavity surface 8a and the parison can extend together with the elastic sheet. Thereby the wall thickness of the molded article is made more uniform.