To provide a technology to derive a distance conversion factor according to a situation.

A pre-processing part 40 sequentially averages GPS speeds contained in obtained positioning data for a predetermined period of time and sequentially measures the obtained number of pulses for the period. A temporary conversion factor deriving part 42 sequentially derives temporary factors for the conversion factor from the number of pulses to a moving distance based on the averaged GPS speed and the measured number of pulses. A filter processing part 46 sequentially performs statistical processing on the derived temporary factors. A correction control part 44 outputs the statistically processed temporary factor or the derived temporary factor as the conversion factor.