A pneumatically operated screw driver ensuring a complete return of a piston to its top dead center. The screw driver has a compressed air return chamber for returning the piston to its top dead center by applying a compressed air in the return chamber to the piston. The piston includes a main piston and an auxiliary piston. The auxiliary piston includes a piston section and a flange section those being disposed in the main piston. The piston section is slidably movable relative to the main piston, and the flange section is axially spaced away from the piston section and radially spaced away from the main piston. After the main piston reaches its bottom dead center, the auxiliary piston further moves toward its bottom dead center. An air space chamber is provided between the piston section and the flange section and within the main piston.