To obtain an apparatus for preparing a foamed thermoplastic resin sheet by which the foamed thermoplastic resin sheet with various thicknesses can be prepd.

An extruder 1 wherein a thermoplastic resin and a foaming agent are melted and kneaded to prepare a molten kneaded substance and is extruded, a die 3 which is provided at the tip end of the extruder l and processes the molten kneaded substance into a sheet-like foamable thermoplastic resin body, a vacuum chamber 4 wherein the foamable thermoplastic resin body extruded from the die 3 is foamed furthermore under vacuum and facing wall parts wherein a pair of wall parts facing each other in the thickness direction of the foamable thermoplastic resin body in the vacuum chamber 4 are formed and at least one of these wall parts comprises a movable top wall part 12 (a movable bottom wall part 13) movable in the approaching direction and in the aparting direction of these both wall parts, are provided.