The present invention relates to an infusion device comprising a housing (10, 12); a compartment inside said housing for positioning a medicament container (78), an infusion needle (124) arranged to said housing, being connectable to said medicament container for delivering a dose of medicament, a piston plunger (72) 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 characterized in that said piston plunger comprises a number of distinct segments (82) being inter-connectable to each other for forming an elongated piston plunger.