A screw length determination system which determines a length of a screw on the basis of an axial position or an amount of rotation of a driver during a period from a time of descent completion to any of a time of provisional seating, an end time of a primary tightening period, and an end time of a primary tightening holding period, in a case in which a time when a tip of the screw comes into contact with a fastening target that is a target for screw tightening is defined as the time of descent completion, a time when a rotation torque of a first motor that causes a rotational movement of the driver around an axis for tightening the screw reaches a first predetermined value is defined as the time of provisional seating, a period during which the rotation torque of the first motor is equal to or greater than the first predetermined value and less than a second predetermined value, and a moving torque of a second motor that causes the driver to reciprocate in an axial direction is a third predetermined value is defined as the primary tightening period, and a period during which the rotation torque of the first motor that causes the rotational movement of the driver around the axis is the second predetermined value, and the moving torque of the second motor that causes the driver to reciprocate in the axial direction is the third predetermined value is defined as the primary tightening holding period, the screw length determination system characterized in that wherein the length of the screw is determined on the basis of a change in the axial position of the driver during the period from the time of descent completion to the time of provisional seating or the end time of the primary tightening period, or wherein the length of the screw is determined on the basis of the amount of rotation of the driver during the period from the time of descent completion to the end time of the primary tightening period or the end time of the primary tightening holding period. in a case in which a time when a tip of the screw comes into contact with a fastening target that is a target for screw tightening is defined as the time of descent completion, a time when a rotation torque of a first motor that causes a rotational movement of the driver around an axis for tightening the screw reaches a first predetermined value is defined as the time of provisional seating, a period during which the rotation torque of the first motor is equal to or greater than the first predetermined value and less than a second predetermined value, and a moving torque of a second motor that causes the driver to reciprocate in an axial direction is a third predetermined value is defined as the primary tightening period, and a period during which the rotation torque of the first motor that causes the rotational movement of the driver around the axis is the second predetermined value, and the moving torque of the second motor that causes the driver to reciprocate in the axial direction is the third predetermined value is defined as the primary tightening holding period, the screw length determination system characterized in that wherein the length of the screw is determined on the basis of a change in the axial position of the driver during the period from the time of descent completion to the time of provisional seating or the end time of the primary tightening period, or wherein the length of the screw is determined on the basis of the amount of rotation of the driver during the period from the time of descent completion to the end time of the primary tightening period or the end time of the primary tightening holding period. <EOS>