To detect a slip state in a contact region of a rotating tire on a road surface, first, the first radical direction data and the second width direction data of measurement data of acceleration at a tread portion of the rotating tire for a duration corresponding to one round of tire rotation are acquired. Second, time series data of acceleration due to tire deformation is extracted from the first radical direction data and displacement data is obtained by subjecting the time series data of acceleration due to tire deformation to a time integration of second order, thereby a deformation at the tread portion is calculated and a contact region of the rotating tire is determined from the calculated deformation. Next, from the second width direction data, a slip region within the determined contact region is specified based on vibration level of the second data.