A injection device in molding machine is capable of efficiently heating and melting a plurality of charged pellets in a short period of time, and injecting a melting resin with superior quality.The injection device has a cylinder including a pellet supply port, a plunger, a driving unit, a melting instrument with a plurality of melting holes, which are cone-shaped passages, formed in a cylinder-shaped instrument body to communicate from inflow-side large openings to outflow-side small openings, a nozzle provided on an injection side of the cylinder and a heating unit heating the melting instrument. A diameter of the tip-end of the plunger is set to be the same as a diameter of the outflow side face of the melting machine.