A low-cost crank type injection device capable of driving an injection screw by a required driving force without using a high output type electric motor. This injection device has a crank mechanism (20) for converting a forward or reverse rotary motion of a driving shaft (21) connected to the output shaft (41) of a servo-motor (40) controlled by a numerically controlled machine (60) through a speed reducer (50) into a reciprocating motion of the injection screw installed on a pusher plate (10), and forwardly drives the injection screw with a high driving force by the servo-motor through the speed reducer and the crank mechanism during the injection process. The servo-motor is driven such that the driving shaft is driven in the forward direction within a range of a given rotary angle corresponding to an operating region excluding the top and bottom dead centers of the crank mechanism, so that abrupt changes in the injection velocity and the rate of change in injection pressure in accordance with the rotary angle of the driving shaft can be prevented.