To make a correction of high accuracy by measuring the diameter change of a vehicle tire by various means using an optical sensor, a pressure sensor, a temperature sensor, and the like, and correcting travel distance of a vehicle according to the diameter of the tire.

An optical sensor LSN optically measures the diameter of a tire of a vehicle and outputs a tire diameter signal, and measures distance between the road surface and an axle and outputs a corresponding signal. A pressure sensor PSN measures the pressure of the vehicle tire and outputs a tire pressure signal, and measures air pressure of the tire and outputs a corresponding signal. A temperature sensor TSN measures outside air temperature. Signals of the optical sensor LSN, pressure sensor PSN, temperature sensor TSN, a GPS sensor and a distance sensor DSN are added to a distance correcting part CNP. The distance correcting part CNP corrects the signal of the distance sensor DSN to measure correctly using the signals of the optical sensor LSN, pressure sensor PSN, temperature sensor TSN, and GPS sensor.