An improved injection molding apparatus having a fixed mold 7 having a molten resin passage 16b and a movable second mold 8 having a molten resin passage 18 is presented in which residual hardened resin left in molten resin passages of the injection apparatus is removed automatically after every injection operation. The invented device is provided with a reciprocating member 34 which is coupled in molten resin passages 16b, 18, and during the injection step, the molten resin passage 16b is extended to the molten resin passage 18 by moving the reciprocating member 34 into the second mold to communicate a mold cavity 17 with a resin injection device 14. After the completion of the injection step, the reciprocating member 34 is moved to the back position to remove any resin left in the molten resin passage towards the first mold. The mold is then opened to separate the fixed mold 7 from the movable mold 8, leaving the reciprocating member 34 in the first mold so as to close off the opening of the molten resin passage 18. When the molds 16b, 18 are separated, this arrangement is effective in preventing exposing the passages 16b, 18 to ambient atmosphere, leading to leaking of molten resin from the passage 16b as well as preventing inclusion of air in molded products to cause a phenomenon called burning.