PURPOSE: To correct a run track according to a shift in initial set position by repeating the operation of only forward or backward movement if the run track shifts from a road in a map.

CONSTITUTION: The run track 100 on a CRT10 is displayed in the order of S1W SN successively from the initial position S and the road 200 is displayed in the order of P1WPN so that a dot corresponding to the S is P and a backward movement point is P-1. If either one occures or every time either of shift keys 32 and 34 is operated, a one-dot correction is made. Namely, an address read of the initial position S(m, n) is made and the data is held in a register in a computer 14. Then, coordinates of the S1 which is the 2nd position behind the initial position S are found. The computer 1 performs arithmetic for finding the P1 and P-1 before and behind the road coordinates P corresponding to the initial position S and corrects the deviation of the run track coordinates S1 easily only by a forward or backward movement command according to the road coordinates P1 or P-1.