Injection device

An injection device for injecting a liquid substance, having a longitudinal axis, a proximal end and a distal end, the injection device comprising a housing and a cartridge holder in which a cartridge with said substance is contained, the housing being provided on the proximal side with a rotatable dose knob and an axially translatable button, the housing comprising:

• a dose setting sleeve adapted to be rotationally engaged with the dose knob,
• a non-rotatable tubular driving element slidingly mounted in the housing, the driving element being engaged with the dose setting sleeve by a threaded connection,
• a non-rotatable axially sliding piston rod adapted to press against a piston located in the cartridge,
• a toothed gear comprising teeth,
• a driving spring,
• wherein the dose setting sleeve is equipped with at least one ratchet arm cooperating with the toothed gear, the piston rod having an external surface comprising at least one row of ratchet teeth, the driving element has a distal side fixedly connected with at least one driving arm cooperating with the ratchet teeth of the piston rod,
• a non-rotatable and axially slidable button sleeve engaged on its proximal side with the button,
• an immovable auxiliary sleeve located within the driving element, and
• wherein the button sleeve is detachably and slidingly engaged with the toothed gear and the button sleeve is slidingly engaged with the auxiliary sleeve by a coupling spring which is compressed when the button sleeve is translated in the distal direction.