A control unit 10 detects the presence/absence of receiving position information based on the position data provided from a GPS receiver 14. While the GPS receiver 14 is supplying no position data, the control unit 10 computes the travel distance of a vehicle according to a vehicle speed pulse signal input from a vehicle speed pulse input unit 18. When the obtained travel distance exceeds a predetermined distance, the control unit 10 determines abnormal reception due to faulty equipment. The predetermined distance is set in advance based on a reception disability area, etc. When the control unit 10 determines abnormal reception due to faulty equipment, it turns on an orange indicator. Thus, the person in the vehicle can recognize the abnormal reception due to faulty equipment.