PURPOSE: To detect a trailer connecting angle continuously and accurately with simple constitution in a semi-trailer towed in the state of a front part of the trailer being supported by a tractor side coupler to which a king pin at the front center of the trailer is locked.

CONSTITUTION: A magnetic scale 5 of circular arc shape with a radius R around a king pin 4 as the center is embedded in a lower face plate 21 of a trailer 2, and a magnetic sensor 7 opposed with a specified clearance to the magnetic scale 5 is fitted to a side face part of a coupler 3 on the tractor 1 side. As the coupler 3 is rotated around the king pin 4 in association with the turning of the tractor 1, the magnetic sensor 7 moves circularly along the lower side of the magnetic scale 5 and passes N-S magnetic field of the magnetic scale 5 in turn so as to generate two sine wave output signals with fills phase shifted by 90°. The change direction and variation of a connecting angle of the trailer 2 are computed and obtained by a computer from these output signals.