To provide a locking pin mechanism which can be turned by hand to a final stage of clamping in a simple mechanism, easily machined and exhibits a bolt locking effect.

An obliquely penetrating lock pin insertion hole 3 is opened in a position slightly separated from the inner surface of a nut 2 with a nominal diameter 1. After inserting a lock pin 5 slightly longer than the length of a lock pin insertion hole 3 and having a pointed tip 4, the nut 2 is turned by hand and fitted to an external thread part 7 of a bolt head 6 and tightened. After reaching a washer 8, a commercially available tool such as a wrench is used to firmly tighten the nut 2 to thereby fix a fixed object 10 held between the washer 8 and a washer 9. Further tightening causes the tip 4 of the lock pin 5 to firmly bite into the external thread part 7 to thereby produce a locking effect. The locking effect can be increased by lengthening the length of the pin, increasing the number of pins inserted, changing the angle of the lock pin insertion hole to the external thread part, and threading the tip of the lock pin.