PURPOSE: To aim at preventing reaction force upon screw fastening, by providing such an arrangement that the positioning is made with the use of a stroke device in accordance with positioning data, and the occurrence of off-centering is determined in accordance with the opening degrees of chuck pawls to issue positional correction data for correcting the off-centering.

CONSTITUTION: Positioning data which are read out from a teaching data memory are temporarily stored in an RAM. The moving distances of X-, Y- and Z-axis movable parts 4, 7, 10 are computed, and X-, Y- and Z-axis drive motors 3, 6, 9 are driven, and then stopped at positions indicated by the data. Then, the opening degrees of chuck pawls 15f through 15i are checked to determine whether the off-centering of a screw with respect to a screw hole exists or not. If the off-centering is determined, position compensating data for aligning the front end of a screw fastener 11 with the center of a screw hole 36a are computed with the use of the data stored in the RAM, and are delivered to X- and Y-axis servo control devices to drive the X- and Y-axis motors 3, 6 so that the positional correction for the automatic screw fastener 11 is made. Thus, upon fastening of a screw the application of unnecessary force to the screw may be eliminated so that the screw is surely fastened.