The invention relates to a torsion spring driven injection device having a dial-down mechanism for dialling down a selected dose size. The torsion spring (2) is strained when setting a dose by rotating a proximally located dose setting button(30) relatively to the housing in a first direction and unstrained when rotating the same dose setting button (30) in a second direction opposite the first direction. In order to secure the torsion spring (2) in its strained position a one-way ratchet mechanism is provided. This one-way ratchet mechanism comprises a fixation element (1) coupled to the housing and having a toothing (6) and a ratchet element (20) having at least one ratchet arm (22) engaging the fixation element (1) and which ratchet element (20) is operational coupled to the dose setting button (30). The dose setting button (30) directly engages the at least one ratchet arm (22) when the dose setting button (30) is rotated in the second direction which releases the at least one ratchet arm (22) from the toothing (6) of the fixation element (1) and allows the ratchet element (20) to rotate relatively to the fixation element (1) in the second direction under influence of the torsion spring (2).Linvention concerne un dispositif dinjection entraîné par un ressort de torsion et comportant un mécanisme réducteur de dose permettant de réduire le volume de la dose choisie. Le ressort de torsion (2) se tend quand on définit une dose en faisant tourner dans un premier sens un bouton proximal de réglage de dose (30) par rapport au logement et se détend quand on fait tourner le même bouton (30) dans un second sens, inverse du premier. Pour maintenir tendu le ressort à torsion (2), linvention comporte un mécanisme à cliquet non réversible. Ce mécanisme à cliquet non réversible comporte, dune part un élément de fixation (1) couplé au logement et pourvu de dentures (6), et dautre part un élément de cliquet (20) dont au moins un bras (22) est au contact de lélément de fixation (
