PURPOSE: To control fastening force without using a tool having a torque sensor.

CONSTITUTION: A bolt and a nut are provided with a mounting-demounting main angular head part and a sub-angular head part 3 which is placed in a step shape on the main angular head part and has an on-specification or off- specification width across flat B1 smaller than the main angular head part and is used only at initial fastening time, and when a tool larger than the width across flat B1 and smaller than a diagonal line length C1 is engaged with the sub-angular head part 3 and is rotated by applying sufficient large torque, respective angle parts 3a of the sub-angular head part 3 are destroyed. The torque applied to a bolt when these respective angle parts 3a are destroyed is determined by a difference between a width across flat of the tool and a width across flat of the sub-angular head part, and since the difference is thought to become almost constant by a tool side tolerance and a bolt side tolerance, bolt fastening force can be controlled within an almost constant range.