To provide a screw part, simply unfastened and reliably fixed without damaging a fundamental function, inexpensively.

Head parts 10 are arranged in two stages in series and besides and engaging parts 20 with which different driver bits are engageable are formed at an upper stag head part 11 and lower stage head part 12. A skirt part 26 having an outside diameter larger than that of the lower head part is formed at the lower stage head part 12 and during fastening, fastening is effected by the driver bit engaged with the upper stage heat part 11. When fastening torque attains a given value, a coupling part 13 is broken and the screw part provided with elasticity by the skirt part 26 and the part is simply fixed at the work without intervening of a washer therebetween. By breakage of the coupling part, approximately constant fastening torque is provided and the screw part is synthetically provided with an effect, such as constant fastening torque, loosening prevention, and torsion prevention. Besides, a screw part is provided at a low cost.