PURPOSE: To shorten the period until the computed position of a construction vehicle converges to the actual position of the vehicle through a Kalman filter by computing the position of the vehicle from the outputs of first and second position detecting means and outputting the computed position through the Kalman filter.

CONSTITUTION: A sensor for dead reckoning corresponding to the assuming section of a Kalman filter is used as a first position detecting means and a GPS sensor is used as a second position detecting means. The navigation sensor is composed of a goniometer 5 for vehicle and wheel rotation sensor 6 and the GPS sensor is composed of an absolute vehicle position sensor 7. The angular velocity of the azimuth of a construction vehicle from the goniometer 5 and the speed of a wheel from the sensor 6 are inputted to the assuming arithmetic section 4 of the Kalman filter. When the second position detecting means does not output a position detecting value yet at the time of performing estimating operation, the sensor fusion by the Kalman filter can be applied while the accuracy is secured, because the then position assumed by the first position detecting means is used in place of the position detecting value.