PURPOSE: To readily and inexpensively manufacture a molded synthetic resin product formed in a spigot shape by employing a mold formed with a stretchable spigot auxiliary mold formed in a bag shape of a film material in the cavity.

CONSTITUTION: An auxiliary mold 1 for a bag-shaped spigot swelled at the end is formed of a film material having small stretchability. Then, an auxiliary mold 1 for a spigot is integrally mounted on the lower mold 8 of a mold 2, and the mold 1 communicates with a pressure tube 4 through the mounting hole 10 of the mold 8 (numeral 7 designates an upper mold of the mold 2, and 3 a cavity). Pressurized fluid is first fed to the interior with the mold to swell the mold 1 in the prescribed shape, synthetic resin 5 is then filled in the cavity 3, is then hardened, the fluid is then removed, the mold is then shrinked, and a molded synthetic resin product 6 having a spigot 9 is removed from the mold 2. When the mold 1 is formed of elastic material such as silicon rubber or the like, the product 6 may be readily released from the mold.