To provide a road surface condition determining device capable enhancing performance and safety of a vehicle by accurately determining a road surface condition between a road surface and tires.

The road surface condition determining device is provided with a speed detecting means of periodically detecting wheel speeds of four wheels of the vehicle, a body speed calculating means of determining a body speed of the vehicle, a slip ratio calculating means of determining a slip ratio from the body speed and an average wheel speed of driving wheels of the wheel speeds, a body acceleration calculating means of determining a body acceleration of the vehicle, a turning radius calculating means of determining a turning radius of the vehicle, a moving average processing means of subjecting the slip ratio and the body acceleration to a moving average process per predetermined time, and a road surface condition determining means of determining the road surface condition from a relationship between a slip ratio and a body acceleration when a turning radius is larger than a predetermined value from data of the slip ratio and the body acceleration subjected to the moving average process.