An injection device in a molding machine comprising: a cylinder having an outlet member formed at a tip-end injection side in a longitudinal direction, a stopper part provided at a rear side, and a pellet supply opening provided in an intermediate position between the stopper part and the outlet member for supplying plastic pellets; a melting device having a plurality of melting holes formed communicating from inflow-side large openings to outflow-side small openings in the longitudinal direction of a device main body, the melting device having a diameter equal to an inner diameter of the cylinder; a heating unit for heating the melting device; and a drive unit for causing the melting device to move in a reciprocating manner, wherein the outflow-side small openings of the melting device face the outlet member, the melting device operates as a plunger in the cylinder, and a return travel of the drive unit is configured to correspond to a melting step of melting the plastic pellets and an outward travel of the drive unit is configured to correspond to an injection step of melted resin, in the cylinder, an opening and closing valve is provided between the outlet member and the melting device, the opening and closing valve is configured to open the outflow-side small openings of the melting device in the return travel step of the melting device and to close the outflow-side small openings of the melting device in the outward travel step of the melting device, characterized in that inflow-side large openings of any adjacent two of the melting holes each have a circular cross section and an inlet part of each of the inflow-side large openings is chamfered to be a plate-shaped chamfered part (22a1) and boundary parts of plate-shaped chamfered parts of adjacent inflow-side large openings are formed like edge. a cylinder having an outlet member formed at a tip-end injection side in a longitudinal direction, a stopper part provided at a rear side, and a pellet supply opening provided in an intermediate position between the stopper part and the outlet member for supplying plastic pellets; a melting device having a plurality of melting holes formed communicating from inflow-side large openings to outflow-side small openings in the longitudinal direction of a device main body, the melting device having a diameter equal to an inner diameter of the cylinder; a heating unit for heating the melting device; and a drive unit for causing the melting device to move in a reciprocating manner, wherein the outflow-side small openings of the melting device face the outlet member, the melting device operates as a plunger in the cylinder, and a return travel of the drive unit is configured to correspond to a melting step of melting the plastic pellets and an outward travel of the drive unit is configured to correspond to an injection step of melted resin, in the cylinder, an opening and closing valve is provided between the outlet member and the melting device, the opening and closing valve is configured to open the outflow-side small openings of the melting device in the return travel step of the melting device and to close the outflow-side small openings of the melting device in the outward travel step of the melting device, characterized in that inflow-side large openings of any adjacent two of the melting holes each have a circular cross section and an inlet part of each of the inflow-side large openings is chamfered to be a plate-shaped chamfered part (22a1) and boundary parts of plate-shaped chamfered parts of adjacent inflow-side large openings are formed like edge.