A screw fastening device (10) includes a base part (11), an extended part (12) with one end (12a) which is attached to the base part in a pivotable manner, a tool part (13) which is fastened to another end (12) of the extended part and makes a screw which is engaged with its front end rotate, a pushing mechanism (15) which pushes the tool part against the screw (14) through the extended part in an axial direction of the screw, a guide part (16) which is fastened to the base part and has an arc shaped part, a torque generating part (17) which generates a torque which makes the extended part rotate along the arc shaped part in a fastening direction of the screw, and a first detection part (18a) which detects when a reaction force of a predetermined value or more acts against the torque generating part.