A driving unit for a portable drug infusion device (10) comprises an electric motor (24) and a mechanical assembly (21, 27, 28, 40) arranged to convert the rotary motion of the motor (24) into a linear motion and to impart said linear motion to the plunger (17) of a syringe (14) mounted outside a body (11) housing the driving unit. The mechanical assembly (21, 27, 28, 40) includes a toothed crown gear (28) driven by a pinion gear (27) integral with a shaft of the motor (24), and an axially sliding rod (21) carrying, at a front end, a pusher (18) associated with the plunger (17) and moving, during its sliding motion, between the inside of the body (11) and the inside of the syringe (14). The rod (21) has a smooth external surface and an axial cavity (21) housing an externally threaded stem (41) integral in rotation with said toothed crown gear (28) and having a rear end portion provided with an internal thread arranged to mesh with the external thread of said stem (41).
