To suppress the occurrence of pitching and yawing in performing traction for forward movement by switching a steering shaft of a trailer so that a caster angle may always become a plus caster relative to its bearing part based on the forward or rearward signal of a tractor, thereby performing self-controlling trailer steering.

This trailer steering device steers a trailer interlockingly in the steering direction of a tractor, whereas a caster angle switching mechanism 20 is formed which switches a steering shaft 5 of the trailer so that a caster angle may always become a plus caster relative to its bearing part 6 based on the forward or rearward signal of the tractor, between the steering shaft 5 of the trailer and the bearing part 6 thereof. The caster angle switching mechanism 20 is formed with an air chamber 14 driving a driving shaft so as to advance and retreat as a switching driving mechanism on a mounting member fixed integrally with the steering shaft 5 around the shaft of the steering shaft 5, and the front end of the operating shaft is connected to a connection part between a link and a connection link 32.