A grip factor estimating apparatus includes a steering torque detecting unit M1, and an assist torque detecting unit M2. When a self-aligning torque estimating unit M6 estimates self-aligning torque generated in front wheels on the basis of detection result of the detecting unit, the quantity of influence of longitudinal force on self-aligning torque is removed on the basis of longitudinal force acting on the front wheels and estimated by a longitudinal force estimating unit M15 and a front wheel slip angle estimated by a front wheel slip angle estimating unit M9y. A grip factor estimating unit M12 estimates the grip factor of the front wheels on the basis of change in self-aligning torque in accordance with the side force.