The invention relates to an actuator for axial displacement of an object. The actuator comprises a cylinder volume having a first portion (6), and an actuator piston disc (5) that is displaceable backand forth in the axial direction in said cylinder volume between an inactive position and an active position, an inlet channel (11) extending between a pressure fluid inlet (12) and the first portion(6) of the cylinder volume, a first inlet valve body (15) arranged in said inlet channel (11), an outlet channel (13) extending between the first portion (6) of the cylinder volume and a pressure fluid outlet (14), and an outlet valve body (27) arranged in said outlet channel (13). The actuator is characterized in that the slave piston (17) is displaceable back and forth in a bore (18) between aninactive position and an active position, and in that the first inlet valve body (15) is constituted by a seat valve body having an inactive position at which the inlet channel (11) is closed, the slave piston (17) being configured to during its movement from the inactive position to the active position ram said first inlet valve body (15) and displace it to an active position at which the inletchannel (11) is open, the outlet valve body (27) being connected to and jointly displaceable with the slave piston (17).