A sunken condition of a vehicle provided with a sensor for measuring air pressure inside a tire, a communication device that carries out wireless communications between a vehicle main body and a tire, a steering torque sensor that measures steering torque, a sensor for measuring rotary speeds of driver and follower wheels of the vehicle or a navigating apparatus including a global positioning system and a dead-reckoning device is detected by a detector having a controller without requiring a sensor for detecting the sunken condition. The controller identifies a sunken condition on the basis of a momentary change in detected data, the status of communication from the sensor, a drop in the measured steering torque, a relative drop of the rotary speed of a follower wheel with respect to that of a driver wheel or a disagreement in measured position data by the navigating apparatus.