PURPOSE: To injection molding an undercut product quickly and accurately by a method wherein molten resin is injected into the mold in which the core made of low temperature melting material is provided, and the resin is released from the mold after its solidification and then the core is eluted out.

CONSTITUTION: The core 6 made of low temperature melting material (e.g. wax family, low temperature melting metal, resin) is fixed to the inside of a movable side frame 4 (numeral 5 is a ram) by way of inserted knock pin 7. Next, a stationary side frame 2 and the movable side frame 4 are clamped. Molten thermoplastic resin is injected at high speed into the cavity 11 formed in between the frame 4 and the core 6 from a nozzle 3 and the cavity is filled with the resin. (numeral 1a is a heating cylinder, 1c is an injecting plunger) Then, after the resin has been solidified, it is released from the mold with the core 11 and is dipped into warm water, whereby the knock pin 7 is drawn out and the core 11 is removed at a softened state or as an eluted liquid. Thus, the undercut product may be obtained.