An injection device (1) for an injectable solution (S) includes one tubular body (3) inside which at least one containment chamber (8,9) for the injectable solution is defined； one dosing reservoir (20) for one fraction of injectable solution (S)； at least one plug (6,7) sliding into the tubular body (3) and including a protruding appendix (16) suitable for entering in the dosing reservoir (20) to eject said fraction of solution； one vent outlet (23) to eject said second fraction of solution. The device (100) includes an accessory (260) with one storing cannula (261) engageable with the vent opening (23), and provided with a first end (266) and a second end (267) between which a storing reservoir (261′) for the second fraction of solution is defined. The device (100), that is particularly suitable for injecting very precise and reduced doses of an injectable solution, is for example of the single-chamber or double-chamber pre-filled type.
