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 (±), 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 immobil
