an expansion molding apparatus for 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 apparatus has a moving device , for example , a cylinder which moles the molds in one orthogonal direction relative to a closing direction of the molds when filling the cavity with the expandable thermoplastic resin beads . after filling the resin beads in the cavity by moving at least one of the molds in at least one direction orthogonal to the closing direction so that a distance in a circumference section of the cavity becomes greater , the mold is moved back to the original position and the application of heat is performed to obtain molded articles .