An injection molding machine of an automatic core change type includes a mold clamping section including stationary and movable platens fitted with first and second matrixes of a mold, respectively, a mold changing robot and a mold stocker severally attached to the mold clamping section and separately controlled by way of a control section, and a temperature controller for controlling the respective temperatures of the first and second matrixes and the mold stocker. As an arm and a chuck move and rotate, respectively, a first cavity unit, attached to the first matrix, is taken out from the first matrix, transported toward the mold stocker, and stored in its corresponding storage hole of the mold stocker. Then, another first cavity unit is taken out from the mold stocker, transported toward the mold clamping section, and attached to the first matrix, whereupon the replacement of the first cavity unit is completed. A second cavity unit is replaced in like manner, whereupon the mold replacement is completed.