To enable detection of a true wet state of a tire by constituting a detecting device out of a mechanism for generating an electric charge continuously, which is provided on the tire or a wheel, and an electromagnetic wave receiving device provided on the carbody side.

An electric charge given to the surface of a tire 1 by an electric charge generator 3 remains on the surface of the tire 1 when the road surface condition is dry, and a discharge is repeated due to the vibration or the like at the time of traveling. An electromagnetic wave accompanying this discharge is detected as a signal by an antenna in a wheel housing and it is judged therefrom that the road surface condition whereon the tire runs is in the dry state. At the time when the road surface condition is wet, an electrostatic charge supplied to the surface of the tire 1 is absorbed by a liquid layer being present between the tire 1 and the road surface and, therefore, it is not given to the surface of the tire 1. Therefore the discharge is not generated either and the electromagnetic wave due to the discharge can not be detected by the antenna even when running is made. It can be judged therefrom whether the tire 1 is running on the wet road surface or whether the tire 1 itself is in the wet state.