A signal acquisition section 19 acquires, for example, a vehicle velocity
pulse signal, which is generated from a vehicle velocity sensor 8 in accordance
with the movement of a vehicle that is a mobile body. A distance information acquisition
section 20 acquires distance information concerning the movement distance
of the vehicle from a GPS receiver 2. A distance factor computing section
21 computes a distance factor value based on the acquired vehicle velocity
pulse signal and distance information. A change detection section 23 then
compares this computed distance factor value and a distance factor reference value
stored in a reference value storage section 30 to detect a continuous change
of the distance factor value. That is, change detection section 23 detects
a change of tire outer diameter due to a change of the pneumatic pressure of the
vehicle's tire or tire exchange, etc. When the change of tire outer diameter is
detected by change detection section 23, a distance factor correction section
26 corrects the distance factor reference value speedily and appropriately.
