Centering of an injection heating cylinder and screw driver is provided even after assembly, using an electric injection mechanism with height adjustment for the screw driver. A front plate is provided with an injection heating cylinder having a screw installed inside at the front middle and a rear plate is provided with are electric motor for injection-driving. The plates are affixed to an installation member. The front and rear plates are linked by guide bars installed between them. The screw driver is inserted into the guide bar and movably disposed between the plates. A driving mechanism connected to an electric motor is composed of a ball screw shaft and a ball nut member, and is installed over the screw driver and the rear plate. A centering member slidably supports the screw driver 10 and, moreover, provides height adjustment, and is installed at the sides of the screw driver.