A screw fastener with simple configuration is provided, which automatically moves a rotary tool such as a nut runner to fastening positions to fasten screws, and the automation is achieved at low thrust. A screw fastener includes: X, Y, and Z-axis linear motion robots that move a nut runner along the directions of X, Y, and Z-axes, respectively, the axes being orthogonal to one another; a reaction force supporting member that supports a main body of the nut runner in a movable manner along each of the directions of the X, Y, and Z-axes and restricts the rotation of the main body around the rotary shaft; and positioning means that positions a body, and thus the screw fastener with simple configuration can automatically move the nut runner to positions to fasten screws, and achieves the automation at low thrust.