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 main piston is first reaches its bottom dead center, and then the auxiliary piston reaches its bottom dead center whereupon screw driving is terminated. Before the auxiliary piston reaches its bottom dead center, and after the main piston moves past a predetermined position, a compressed air is introduced into the return chamber. After the main piston reaches the bottom dead center, application of the compressed air pressure in the return chamber to the auxiliary piston is prevented.