A pneumatically operated screw driver includes a mechanism for assisting separation of screw from a screw band. The band is made of a deformable material to which a plurality of screws are detachably held. The band has one side where a head of the screw is exposed, and has another side from which a threaded portion of the screw extends. The screw driver includes a housing, a slide member, a pneumatic motor and a drive bit driven by the pneumatic motor and movable in an axial direction thereof and rotatable about the axis. The slide member is slidable in the axial direction, and is formed with a screw passage. The other side of the screw band is pressed onto a wall of the slide member. Shearing deformation occurs in the screw band when the drive bit presses the head of the screw while the other side of the screw band is held by the wall, thereby separating the screw from the screw band. A sub-piston cylinder is provided beside the housing and, a sub-piston connected to the slide member is disposed in the cylinder. A compressed air is introduced into a sub-piston chamber from the pneumatic motor to move the sub-piston, thereby moving the slide member at a phase of separation of the screw from the screw band.