Injection device for delivering a defined number (n) of equal doses of a fluid substance contained in a reservoir (P), the device having a housing (1) within which an arming mechanism and a dose delivery mechanism are arranged along the longitudinal axis of the housing, the housing (1) being coupled to an enclosure (3) adapted for receiving the reservoir (P) with the fluid substance, the arming mechanism comprising a setting sleeve (5) which is axially non-displaceable and rotatable around the axis of the housing in two opposite directions by a defined setting angle (a), the setting sleeve (5) being coupled with a spring (10) which is strained by the rotation of the setting sleeve (5) during arming of the device； the dose delivery mechanism comprising a screw ring (6) and a piston rod (4) which is non-rotatable and axially displaceable within the setting sleeve (5), the piston rod (4) cooperating with the screw ring (6) so that during arming of the device the screw ring (6) and the piston rod (4) are immobilized, and during delivery of each dose the piston rod (4) is displaced along the housing (1) by a defined distance due to unwinding of the spring (10) and rotation of the screw ring (6), the displacement of the piston rod (4) causing the fluid substance to be discharged from the reservoir (P). The injection device according to the invention 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 (5) on its inside surface, and at least one longitudinal arm (4.2) located at the proximal end of the piston rod (4), 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
