To efficiently check road information added to a road map based upon digital map data by automatically judging a place where a road branches off.

Parallax image data A and B obtained by taking photographs at two nearby points are selected out of road photography parallax image data obtained by continuous or intermittent photography and 1st identical blocks B11 and B12 and 2nd identical blocks B21 and B22 are set in both the parallax image data; when the isolation distance d1 between the 1st identical blocks B11 and B12 and the isolation distance d1 between the 2nd identical blocks B21 and B22 are not equal to each other, it is decided that both selected parallax image data correspond to a branch point of the road and when d1=d2, it is decided that both selected parallax data do not correspond to the branch point of the road.