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.