A hollow body molding device that forms a hollow molded body in a main cavity (A) of a mold having in its one end a pressure port including a floating core (18; 24) by injecting molten resin through an injection gate (F) into the main cavity (A) having an outlet (A1) in its other end, and then pressure-injecting pressurized fluid through the pressure port into the main cavity (A) to move the floating core (18; 24) toward the outlet (A1) through the molten resin injected into the main cavity (A) so as to extrude the molten resin through the outlet (A1) and discharge the extruded molten resin into a waste cavity (B), the hollow body molding device comprising: a floating core accommodating portion (D) that is located between the outlet (A1) of the main cavity (A) and a communication passage (C) leading to the waste cavity (B) and adjoins the main cavity (A) and that accommodates the floating core (18; 24) moved to the outlet (A1) by the pressurized fluid; and a core receiving portion that has a diameter (d2) smaller than an outside diameter (d3) of the floating core (18; 24) and that is located between the communication passage (C) and the floating core accommodating portion (D) to receive the floating core (18; 24) moved to the outlet (A1) and discharged from the main cavity (A) by the pressurized fluid, wherein an inside diameter (d4) of the floating core accommodating portion (D) is larger than the outside diameter (d3) of the floating core (18; 24), and when the floating core (18; 24) is received by the core receiving portion , the floating core (18; 24) is completely contained in the floating core accommodating portion (D) and communication between the floating core accommodating portion (D) and the communication passage (C) is cut off, characterized in that the diameter (d1) of the main cavity (A) is continuously the same in the longitudinal direction thereof, and the inside diameter (d4) of the floating core accommodating portion (D) is smaller than the diameter (d1) of the main cavity (A). a floating core accommodating portion (D) that is located between the outlet (A1) of the main cavity (A) and a communication passage (C) leading to the waste cavity (B) and adjoins the main cavity (A) and that accommodates the floating core (18; 24) moved to the outlet (A1) by the pressurized fluid; and a core receiving portion that has a diameter (d2) smaller than an outside diameter (d3) of the floating core (18; 24) and that is located between the communication passage (C) and the floating core accommodating portion (D) to receive the floating core (18; 24) moved to the outlet (A1) and discharged from the main cavity (A) by the pressurized fluid, wherein an inside diameter (d4) of the floating core accommodating portion (D) is larger than the outside diameter (d3) of the floating core (18; 24), and when the floating core (18; 24) is received by the core receiving portion , the floating core (18; 24) is completely contained in the floating core accommodating portion (D) and communication between the floating core accommodating portion (D) and the communication passage (C) is cut off, characterized in that the diameter (d1) of the main cavity (A) is continuously the same in the longitudinal direction thereof, and the inside diameter (d4) of the floating core accommodating portion (D) is smaller than the diameter (d1) of the main cavity (A). <EOS>