PURPOSE: To prevent a seating point from slippage and to stabilize the fastening axial force, by calculating the seating point with the torque rate between two points of a wider angle range at the torque rate side of the latter half of the torque rate changing point which is the starting point of the latter half torque rate and a 4th set point.

CONSTITUTION: A former half torque rate RTA=(T2-T1)/a is found with the fastening angle (a) between 1st and 2nd set points that fastening torques T1, T2 differ in the specific quantity at the fastening time former half being measured by an encoder 5. Then, the latter half torque rate RTB=(T4-T3)/(c-b) is found with the fastening angle between 3rd and 4th set points that the latter half torques T3, T4 differ in the specific quantity at the fastening time latter half being measure by the encoder 5. The fastening angle and torque at the torque rate change point are found thereby, a logical seating point is calculated from the 1 torque rate between the torque rate change point and 4th set point and the fastening is performed until reaching the specific angle from the seating point.