PURPOSE: To shorten a manufacturing process and to accurately take the position of a parting surface, by bonding one end of a flexible membrane to the whole of the parting surface of a master model while bonding the other end thereof to the inner peripheral part of a frame body for an injection mold and covering the front and rear surfaces of the master model with the aforementioned membrane before injecting and solidifying a flowable resin to form male and female molds.

CONSTITUTION: Support pins 14 are arranged in a frame body 13 for a box-shape injection mold. One end of a flexible membrane 12 is bonded to the whole of the parting surface 15 of a master model 11 and the other end thereof is bonded to the inner peripheral part of the frame body 13 for the injection mold, and the front and rear surfaces of the master model 11 are covered with the flexible membrane 12 and, thereafter, the master model 11 is placed on the support pins 14. A flowable substance 5 is injected in a male mold part 10 by the opening and closing the valve 7 provided to an inflow port branch point 8 to be solidified and, thereafter, the flowable substance 5 is injected in a female mold part 9 to be solidified. The master model 11 has a piercing hole 11b and the flowable substance injected in the female mold part 9 also fill the piercing hole 11b.