To provide the injection device of a molding machine with simple constitution such as a die casting machine or an injection molding machine.

The injection device 1 includes: an injection cylinder device 7 driving an injection plunger 5; a conversion cylinder device 9 communicating with the injection cylinder device 7 so as to feed a hydraulic fluid to the injection cylinder device 7; and a driving device 11 capable of making the conversion cylinder device 9 be extended and contracted. The injection cylinder device 7 includes: an injection piston 17; a booster piston 18; and an injection cylinder tube 15 storing them. The booster piston 18 includes a small diameter part 18a and a large diameter part 18b. The injection cylinder tube 15 includes: an injection cylinder part 15a where the injection piston 17 and the small diameter part 18a slide; and a booster cylinder part 15 where the large diameter part 18b slides. A conversion head side chamber 21h in the conversion cylinder device 9 communicates with the injection head side chamber 15h and a booster head side chamber 15g on the side opposite to the injection piston 17 of the booster piston 18.