To provide a screw fastening device automatically starting the driving of a driver tool, if the comparison of the position information of the driver tool and the position information of a target screw fastening point agree.

The screw fastening machine 1 is constituted to make a worker grip the driver tool 4 to operate above a workpiece, and lower the driver tool 4 to a target screw fastening point to carry out screw fastening work, includes: position detector 14, 23, 34 detecting the position information of the driver tool 4; and a control unit 50 constituted so as to store the position information of a target screw fastening point in advance, and issue a driving start instructing signal to the driver tool 4 when the position information of the screw fastening point and the driver tool 4 is compared and the comparison agrees, during operation of the driving tool 4 above the workpiece.