PURPOSE: To ensure the sure action of a lock mechanism as well as reducing its operating force by improving the structure of the lock mechanism for restricting spread-opening of a pair of jaws, which hold a king pin, in the case of a coupler which connects a tractor to a trailer.

CONSTITUTION: A guide groove 3a for guiding a king pin P of a trailer is formed in the rear part of a coupler base 3 supported to a tractor rear part, and a pair of right and left jaws 4 for clamping the king pin P are pivotally mounted to the front end of the groove 3a by pivots 5. Spread-opening of the paired jaws 4 is restricted in a condition that the jaw is surrounded by a lock plunger 7 advanced and retracted by operating an operating lever 15. Here a lock mechanism R for preventing a movement of the lock plunger 7 in a non-locked position is composed of the first link 11 for connecting the lock plunger 7 to the operating lever 15, second link 12 for connecting intermediate part of the first link 11 to the coupler base 3 and a stopper 20 possible to restrict movement of both links 11, 12.