PURPOSE: To provide a trailer by which aligning can surely be carried out in the rectilinear advance direction by controlling rotation of a drag rod by an aligning device and the number of part items can be reduced and a structure can be simplified and incorporation and adjustment can be facilitated and maintenance can be improved as a whole.

CONSTITUTION: An aligning device 52 is arranged on a trailer body 1 so as to carry out aligning always in the rectilinear advance direction by controlling rotation of a drag rod 21. The aligning device 52 has a vertical directional support shaft 53 installed on the trailer body 1, and an intermediate part of a pair of aligning arms 54 arranged opposite to each other on this support shaft 53 is pivoted rotatably freely. A compression spring 69 is arranged/ supported between one end parts of the pair of aligning arms 54. An aligning roller 18 interlocked with rotation of the drag rod 21 is engaged between the other end parts of the pair of aligning arms 54. By the compression spring 69, the pair of aligning arms 54 are energized in the direction for sandwiching always the aligning roller 18 between them.