Injection device for delivering a defined number (n) of equal doses of a fluid substance contained in a reservoir, the device having a housing within which an arming mechanism and a dose delivery mechanism are arranged along the longitudinal axis of the housing, the housing being coupled to an enclosure adapted for receiving the reservoir with the fluid substance, wherein - the arming mechanism comprises a setting sleeve which is axially non-displaceable and rotatable around said axis of the housing in two opposite directions by a defined setting angle (α), the setting sleeve being coupled to a spring the proximal end of which is coupled to the housing so as to be strained by the rotation of the setting sleeve during arming of the device; - the dose delivery mechanism comprises (i) a screw ring having an internal thread (6.1), (ii) a piston rod having an external thread (4.0) and at least one longitudinal groove (4.1), and (iii) an immobilized blocking sleeve having at least one blocking projection (7.1) engaging with the groove (4.1) to prevent rotation of the piston rod, which is axially displaceable within the setting sleeve, - the screw ring is selectively engaged in rotation with the setting sleeve by means of projections (6.3) arranged on the internal surface of the screw ring in equal distances defined by the setting angle (α) and of an elastic arm (5.4) located on the distal end of the setting sleeve, the elastic arm (5.4) engaging one of said projections (6.3) after each rotation by the setting angle (α), wherein during arming of the device the screw ring and the piston rod are immobilized by means of a trigger mechanism (12, 13) located on the housing during delivery of each dose the trigger mechanism (12, 13) is released allowing rotation of the screw ring and displacement of the piston rod along the housing by a defined distance due to unwinding of the spring, the displacement of the piston rod causing the fluid substance to be discharged from the reservoir, characterized in that the device is provided with blocking means preventing arming of the device for delivery of a subsequent dose after said defined number (n) of doses of a fluid substance have been delivered, the blocking means comprising at least one longitudinal protrusion (5.1) located near the distal end of the setting sleeve on its inside surface, and at least one longitudinal arm (4.2) located at the proximal end of the piston rod, the at least one longitudinal protrusion (5.1) and the at least one longitudinal arm (4.2) mutually blocking each other after said defined number (n) of doses of a fluid substance have been delivered which prevents rotation of the setting sleeve by the entire setting angle (α) in the arming direction. - the arming mechanism comprises a setting sleeve which is axially non-displaceable and rotatable around said axis of the housing in two opposite directions by a defined setting angle (α), the setting sleeve being coupled to a spring the proximal end of which is coupled to the housing so as to be strained by the rotation of the setting sleeve during arming of the device; - the dose delivery mechanism comprises (i) a screw ring having an internal thread (6.1), (ii) a piston rod having an external thread (4.0) and at least one longitudinal groove (4.1), and (iii) an immobilized blocking sleeve having at least one blocking projection (7.1) engaging with the groove (4.1) to prevent rotation of the piston rod, which is axially displaceable within the setting sleeve, - the screw ring is selectively engaged in rotation with the setting sleeve by means of projections (6.3) arranged on the internal surface of the screw ring in equal distances defined by the setting angle (α) and of an elastic arm (5.4) located on the distal end of the setting sleeve, the elastic arm (5.4) engaging one of said projections (6.3) after each rotation by the setting angle (α), wherein during arming of the device the screw ring and the piston rod are immobilized by means of a trigger mechanism (12, 13) located on the housing during delivery of each dose the trigger mechanism (12, 13) is released allowing rotation of the screw ring and displacement of the piston rod along the housing by a defined distance due to unwinding of the spring, the displacement of the piston rod causing the fluid substance to be discharged from the reservoir, during arming of the device the screw ring and the piston rod are immobilized by means of a trigger mechanism (12, 13) located on the housing during delivery of each dose the trigger mechanism (12, 13) is released allowing rotation of the screw ring and displacement of the piston rod along the housing by a defined distance due to unwinding of the spring, the displacement of the piston rod causing the fluid substance to be discharged from the reservoir,