To provide an easily machinable pin mechanism for locking allowing manual turning until a tightening final stage by a simple mechanism, and exerting an anti-loosening effect for a bolt.

A lock pin hole composed of an upper hole 3 of a length to the middle of a thickness of a nut 2 and a lower hole 4 having a diameter 80% of the upper hole 3 is bored, slightly separated from an inner face of the nut 2 of a nominal diameter 1, and by inserting a pin 5 longer than the upper hole length, manually turning the nut 2 on a male screw part 7 of a bolt head 6 and fitting and tightening the nut 2 until reaching a washer 8, and strongly tightening it by using a tool such as a wrench, a fixing object 10 held between the washer 8 and a washer 9 is fixed. By further tightening, the pin 5 is further inserted into the lower hole 4 with a diameter smaller than a pin diameter while broadening the lower hole 4, and by inflating a nut inner face of a lower hole insertion part of a lock pin tip is inflated to generate anti-loosening pressure. The anti-loosening effect can be increased by increasing the pin length, increasing the number of pin holes, and slanting the lock pin hole.