The apparatus of the invention is an injection molding apparatus of a type adapted to cool a molten metal under shearing by an extrusion screw in a substantially vertical chamber into a semi-solidified slurry and then inject the semi-solidified slurry discharged from a discharge port at the lower end of a channel into molding plates, in which a clamping device is adapted to open or close a movable plate relative to a stationary plate in the horizontal direction, and a connection member having, at the inside, a vertical first channel and a second channel extending in the horizontal direction from the lower end of the first channel and in communication with the stationary plate is connected to the discharge port at the lower end of the chamber. Since this can inject and mold light metal molding products of high quality with less pore or shrinkage without excessively enlarging the size for the height of the apparatus, casting products of high quality can be obtained at a reduced cost by injection molding.