PURPOSE: To accurately measure a traffic volume by recognizing the existence of parallel vehicles surely and detecting the vehicles separately when a picture which indicates the existence of the parallel vehicles in an image picking up picture exists.

CONSTITUTION: When a part of a parallel traveling small vehicle 12 which travels the adjacent lane of a discriminating object large vehicle 11 is hidden for instance, a position on the measuring area plane (x) axis of a feature point C of the vehicle 12 observed while overlapping at a time (t) is (a) and on an optical axis 1 which connects the point(a) and a camera center line, actual feature points C and G of the automatic vehicle are present together. On the other hand, in the condition of a time t', the feature points C and G advance up to the position indicated with a C' and a G' respectively. Then, optical axes 2 and 3 which connect a camera center and the points C' and G' respectively become different line segments and also the positions on a measuring area face become different points, points (c) and (d), respectively. Thus, by discriminating whether the feature point is the head part of the parallel traveling vehicle or not, the vehicles observed while overlapping can be detected separately.