To provide a method for insert molding which enables the implementation of insert molding in a state in which an insert part is held stably at a prescribed position in a cavity.

A method for insert molding using a mold assembly having a fixed mold part 10, a movable mold part 11, and a set of insert part supporting devices 20 arranged outside a cavity 12 includes each process in which both ends of the insert part are gripped by the respective supporting devices 20, when the fixed mold part 10 and the movable mold part 11 are clamped together, the respective supporting devices 20 are moved to be separated from each other so that the insert part is held in the cavity 12 with tension applied to the insert part, and a molten resin is introduced into the cavity 12.