An injection unit of an injection molding machine is provided, in which an offset load is not applied to a ball screw (6) for moving an injection screw (13). A pusher plate (5) is provided that is moved while being guided by guide rods (4) extending between front and rear plates (2, 3), and that supports the injection screw (13) and the ball screw (6) so that they are rotatable but axially immovable with respect to the pusher plate (5). The ball screw (6) is threadedly engaged with a ball nut (7) mounted to the rear plate (3) so as to be unrotatable and axially immovable with respect to the rear plate (3). A support plate (20) is coupled through coupling rods (21) to the pusher plate (5). The ball screw (6) has another end supported on the support plate (20) in a rotatable but axially immovable manner with respect to the support plate (20). The pusher and support plates (5, 20) are supported on a base by linear motion guides (22, 23). The ball screw (6) is supported at its two ends by the pusher and support plates (5, 20) so as not be tilted, whereby no offset load is produced and the service life of the injection unit is prolonged.