To provide a rear wheel steering control device for a trailer tractor vehicle whereby a connection angle between the tractor vehicle and a trailer is prevented from being excessively generated at turning time by a sharp angle, steering performance thereof can be enhanced.

A turn control means, detecting a connection angle between a tractor vehicle and a trailer to steer a rear wheel in the same direction to a front wheel of the tractor vehicle when the connection angle is a prescribed value or more, is provided. When the connection angle is less than the prescribed value, the rear wheel is held in a neutral position. Further, in accordance with a connection angle ψ (S4) and a steering angle δf(S6), as a formula δr=tan-1 [(L2-Scosψ)tanδf-L1sin...]/[L2+L1cosψ-Scosψ], a steering angle δr of the rear wheel is determined (S7). Where, L1 is a distance between front/rear axles of the tractor vehicle, L2 is a distance from a connection point to an axle of the trailer, and S is an offset distance between a rear wheel axle and the connection point.