To provide an injection molding apparatus using a simple mold capable of being rapidly and simply altered in its cavity shape corresponding to the shape of a molded product and capable of sufficiently satisfying the demand related to the production of a small lot, the shortening of the time limit of delivery and the decrease in cost.

The injection molding apparatus 1 is equipped with the simple mold 2 having a replacing part 14, which has a cavity 20 formed thereto, and the fixing part 15a fixed to a machine main body part 4, and a control part 5 for controlling the opening/closing operation of the simple mold 2. The replacing part 14 has the fixed template 16 fixed to the fixing part 15a, the movable template 17 approaching and separating with respect to the fixed template 16 and an ejector mechanism 18 for ejecting a molded product 25. A hot runner 12 for sending out a molten resin 3 to the cavity 20 in a molten state is formed to the fixing part 15a and a cold runner 21 for guiding the molten resin 3 sent out of the hot runner 12 to the cavity 20 is formed to the fixed template 16.