A threaded member tightening tool having a function of temporarily tightening a threaded member, the threaded member tightening tool comprising: a drive unit rotationally driving a threaded member engaging unit engageable with the threaded member; a torque detecting device configured to detect a load torque applied to the threaded member engaging unit; and a control unit controlling drive of the drive unit; wherein the control unit controls the drive unit such that the threaded member engaging unit is first rotated forward by a first rotation quantity and then automatically rotated reversely by a second rotation quantity less than the first rotation quantity; and characterised in that the control unit allows selection between: a time control mode in which the control unit controls a quantity of rotation of the threaded member engaging unit by controlling a drive time of the drive unit such that the control unit first drives the drive unit forward for a first period of time, thereby rotating the threaded member engaging unit forward by the first rotation quantity, and then drives the drive unit reversely for a second period of time, thereby rotating the threaded member engaging unit reversely by the second rotation quantity, the first period of time being set so that the first rotation quantity is a rotation quantity at which the threaded member will not be seated on a to-be-fastened member; and a torque control mode in which the first rotation quantity is a rotation quantity by which the threaded member is rotated so that the threaded member is tightened by a predetermined tightening torque after being seated on a to-be-fastened member, and in which the threaded member engaging unit is first rotated by the first rotation quantity, thereby seating the threaded member on the to-be-fastened member and further applying a predetermined tightening torque to the seated threaded member, and then the threaded member engaging unit is automatically rotated reversely by the second rotation quantity. a drive unit rotationally driving a threaded member engaging unit engageable with the threaded member; a torque detecting device configured to detect a load torque applied to the threaded member engaging unit; and a control unit controlling drive of the drive unit; wherein the control unit controls the drive unit such that the threaded member engaging unit is first rotated forward by a first rotation quantity and then automatically rotated reversely by a second rotation quantity less than the first rotation quantity; and characterised in that the control unit allows selection between: a time control mode in which the control unit controls a quantity of rotation of the threaded member engaging unit by controlling a drive time of the drive unit such that the control unit first drives the drive unit forward for a first period of time, thereby rotating the threaded member engaging unit forward by the first rotation quantity, and then drives the drive unit reversely for a second period of time, thereby rotating the threaded member engaging unit reversely by the second rotation quantity, the first period of time being set so that the first rotation quantity is a rotation quantity at which the threaded member will not be seated on a to-be-fastened member; and a torque control mode in which the first rotation quantity is a rotation quantity by which the threaded member is rotated so that the threaded member is tightened by a predetermined tightening torque after being seated on a to-be-fastened member, and in which the threaded member engaging unit is first rotated by the first rotation quantity, thereby seating the threaded member on the to-be-fastened member and further applying a predetermined tightening torque to the seated threaded member, and then the threaded member engaging unit is automatically rotated reversely by the second rotation quantity. a time control mode in which the control unit controls a quantity of rotation of the threaded member engaging unit by controlling a drive time of the drive unit such that the control unit first drives the drive unit forward for a first period of time, thereby rotating the threaded member engaging unit forward by the first rotation quantity, and then drives the drive unit reversely for a second period of time, thereby rotating the threaded member engaging unit reversely by the second rotation quantity, the first period of time being set so that the first rotation quantity is a rotation quantity at which the threaded member will not be seated on a to-be-fastened member; and a torque control mode in which the first rotation quantity is a rotation quantity by which the threaded member is rotated so that the threaded member is tightened by a predetermined tightening torque after being seated on a to-be-fastened member, and in which the threaded member engaging unit is first rotated by the first rotation quantity, thereby seating the threaded member on the to-be-fastened member and further applying a predetermined tightening torque to the seated threaded member, and then the threaded member engaging unit is automatically rotated reversely by the second rotation quantity.