To provide a distance coefficient calculator which detects changes in a moving state such as an exchange of tires, a change in the air pressure of tires, and fitting and removal of chains automatically and with high precision, and does not perform erroneous correction of a pulse distance coefficient, even in an environment where the reliability of a distance measuring portion of a GPS or the like is low.

On the basis of the output of an air pressure sensor 8 for a tire being a sensor for detecting a change in the state of a vehicle, and the error of a pulse distance coefficient K1 calculated by a pulse distance coefficient error calculating portion 5 from a travel distance error calculated by a GPS satellite position measuring portion 1, a state change detecting element 6 detects the state change, and switches the time constant of a filter.