To provide a key having the capability of fully exhibiting a theft preventing effect by installing a locking mechanism part having a turning member operated with the key in an engagement part with a communication opening on the end of a king pin, regarding a lock fitted to a trailer king pin for prohibiting the use thereof.

For a locking purpose, a key 10 is positioned under the king pin 2 of a trailer, and moved upward, so as to house a king pin end part within the cavity 42 of an engagement part 12. Then, when the engaging groove 4 of the king pin 2 arrives at such a position as corresponding to the engagement member tip end part 52a of the key 10, an engagement member 52 projects into the engaging groove 4, thereby providing a locked state. For an unlocking purpose, the key 10 is kept fit to the keyhole structure 60 of a locking mechanism part 14, and an operation rod 60a is rotated with the key 10, thereby rotating a lever member 64. At this rotating operation process, a pin 66 comes in contact with cam surface 52d, and after the engagement member 52 is moved in a direction for parting from the king pin 2, the lock 10 is moved downward.