PURPOSE: To obtain a usual turning characteristic even though inferior steering control occurs, by compensating a steered value of steerable one of an auxiliary front steering mechanism and a rear wheel steering mechanism for an error in turning degree if either the auxiliary front wheel mechanism or the rear wheel steering mechanism becomes inferior in the steering control operation.

CONSTITUTION: When one of an auxiliary front wheel steering mechanism 5 and a rear wheel steering mechanism 6 becomes inferior in steering control operation but the other one of them is normal, a compensating means 8 compensates a desired steering value in the steering mechanism whose operation is normal in order to make up for an error in a desired steering value, caused by the steering mechanism whose operation is inferior. For example, if rear wheels cannot be steered so that they are set in a straight running condition in the case of steering the front and rear wheels with reverse phases, the compensation is made such that the steering value for the front wheels are increased in order to prevent the turning radius from increasing. Meanwhile, if the rear wheel cannot be steered so that they are in a straight running condition in the case of the steering the front and rear wheels with the same phase, the compensation is made such that the steering value for the front wheel is reduced in order to prevent the turning radius from decreasing.