PURPOSE: To provide a rear side rear axle holding device of simple constitution preventing reverse steering at the time of moving backward without spoiling self-steering function.

CONSTITUTION: A rear side rear axle holding device for a two rear axle vehicle is provided with a gear position sensor 16 for outputting a signal corresponding to the proceeding direction of the vehicle, an expansion means 14 for changing the length of a torque rod 13 for holding a rear side rear axle 8, and a controller 15 for actuating the expansion means 14 on the basis of the output of the gear position sensor 16 so as to variably control the caster angle of the rear side rear axle 8. As a result, the caster angle of the rear side rear axle 8 always has the positive value in relation to the proceeding direction whichever forward/ backward direction the vehicle moves.