PURPOSE: To prevent biting of a king pin plate at the upper surface of the coupler, by a method wherein a spherical bearing is mounted at the inside of a coupler body, and the coupler is caused to revolve horizontally as it is caused to pitch and roll.

CONSTITUTION: When a coupler plate 11 and a king pin plate 4 are coupled monolithically by means of a king pin 5, a revolving movement is difficult to be produced between the coupler plate 11 and the king pin plate 4 during the revolution, and is produced between an inner race 13 and an outer race 14 of a spherical bearing 18 located between the two plates 11 and 14 which have an extremely less movement resistance. Additionally, pitching and rolling movements can be freely performed because said section is formed in a sphericity. Furthermore, when it is not coupled with a trailer 2, the coupler 11 and a channel 23, whereinto the king pin 5 is inserted, are always maintained in a specified position through the action of a spiral spring 19, and the coupler plate 11 always restores to a constant position, whereby coupling with the king pin 5 can be facilitated.