A GPS receiver is used to receive a signal from GPS satellites, and a current vehicle position is detected based on the received signal. A control unit estimates the current vehicle position based on a travel speed of a vehicle from a vehicle speed sensor and a travel direction of the vehicle from a direction sensor. After correcting the estimated current vehicle position based on the current vehicle position from the GPS receiver, a power supply to the GPS receiver is turned off, and thereafter an error of the estimated current vehicle position is calculated based on an error accumulated for a certain time period regarding the speed and the direction. When the error of the estimated current vehicle position exceeds an error tolerance, the power supply for the GPS receiver is restarted to correct the estimated current vehicle position.