In order to provide a method for accurately determining the road surface state even when the tire state and external information change, when determining the state of the road surface from the time-varying waveform of vibration of a tire during travel detected by a vibration detection means, in addition to the vibration waveform of the tire, tire state information and external information inputted to the tire are acquired, and a determination parameter for determining the road surface state calculated from the tire vibration waveform is corrected or modified on the basis of the acquired tire state information or external information, and either the road surface state is determined using said corrected or modified determination parameter, or the road surface state is determined from the determination parameter and the state information or external information.
