The present invention relates to an infusion device comprising a housing； a compartment inside said housing for positioning a medicament container, an infusion needle arranged to said housing, being connectable to said medicament container for delivering a dose of medicament, a piston plunger arranged in said housing capable of acting on said medicament container for delivering a dose of medicament, mechanical drive means capable of acting on said piston plunger for delivering a dose of medicament. The invention is characterised in that said piston plunger comprises a number of distinct segments being inter-connectable to each other for forming an elongated piston plunger.
