PURPOSE: To provide a lock for a kingpin capable of thoroughly exerting a theft preventing effect and having a lightweight and simple structure at a low cost.

CONSTITUTION: This lock for a kingpin is provided with the first arm member 12 having an arm section 13a to be coupled with the coupling groove of the kingpin of a trailer and opening formation sections 13b, 13c and the second arm member 14 having a similar arm section 15a and opening formation sections 15b, 15c. The first and second arm members 12, 14 are connected by a hinge inserted with a pin 16 into the openings 13b', 15c' of the opening formation sections 13b, 15c. Both arm members 12, 14 are hooked by a hooking means including a main body section 18A in the opening 15b' of the opening formation section 15b and an auxiliary section 18B in the opening 13c' of the opening formation section 13c. The arm members 12, 14 are hooked when a projection 19b is protruded from the main body 1a of the main body section 18A and inserted into the recessed hole 19c of the auxiliary section 18B, and they are unhooked when the main body 19a is operated by a key 22.