A drug delivery device comprising a housing (1) with a drug container (2) being provided with delivering means comprising a piston (5) for expelling a drug (4) from the drug container (2). The housing (1) further comprising a displaceable piston rod (6) abutting the piston (5) of the drug container (2) and rotating means being in engagement with the piston rod (6), said rotating means being provided with a one-way mechanism. The one-way mechanism comprises a helical spring (12) wound tightly around an axle (11), one end of said helical spring (12) being fixed in relation to the housing (1), the other end of the helical spring (12) being in a free state.