To provide a fun-flat state detecting method for a run-flat tire capable of detecting a run-flat state of the run-flat tire, thus securing safe driving.

In this method for detecting the run-flat state if tire air pressure drops in the run-flat tire of which a core is provided on the periphery of a wheel of a vehicle and supports a load of the vehicle, based on wheel speed information obtained from a wheel speed detecting means provided in relation to the wheels of the vehicle, vibration components of the run-flat tire due to roughness on a road surface on which the vehicle is traveling are detected. After that, based on the comparative result between the vibration components and the vibration components when the run-flat tire travels at a predetermined normal tire air pressure, it is judged whether or not the run-flat tire is in the run-flat state.