PURPOSE: To provide a screw clamping device which can automatically increase a pressing force of a screw clamping bit in compliance with increase of a torque.

CONSTITUTION: A ball screw nut 2a is rotationally connected to an output shaft 1a of an electric driver 1, a ball screw shaft 2b is connected to a ball screw nut 2a via balls, and when tone ball screw nut 2a is rotated in the clamping direction of the screw, the ball screw shaft 2b is advanced in the forwarding direction of the screw. A compression spring 4 is provided between the upper end of the ball screw shaft 2b and the ball screw nut 2a so as to change the forwarding force of the ball screw shaft 2b. A screw clamping bit 3 is connected to the ball screw shaft 2b and is rotationally supported. And component force in the pressing direction is regulated by the compression spring 4 so that the screw clamping bit 3 increase/decrease the pressing force against the screw in proportion to the increase/decrease of the clamping torque.