a method of forming molded articles having consistently or partially thin - walls by expanding expandable thermoplastic resin beads in a cavity formed between molds with the application of heat . the introduction of the resin beads is carried out after causing the width of a circumference section of the cavity to be greater than the diameter of the beads by moving one of or both of the molds in at least one direction orthogonal to a direction of closing the molds , the molds are then moved back to the normal positions , and the introduced resin beads are heated to form the molded article . also , an expansion molding apparatus having a moving device such as a cylinder which moves the molds in at least one direction orthogonal to the closing direction during the introduction of the expandable thermoplastic resin beads , whereby consistently or partially thin - walled molded articles are manufactured .