PURPOSE: To easily and exactly grasp the change of a lane from the change condition of a pattern by grasping the moving condition of vehicle in the lane from the pattern showing the presence/absence of a vehicle in a measurement sampling area.

CONSTITUTION: On a road surface in lanes 10, 12 and 14, plural measurement sample areas 16 are successively set along the traveling direction of a vehicle 2. Next, the presence/absence of the vehicle 2 in each measurement sample area is decided for each prescribed time. Further, the result of deciding the presence/absence of the vehicle 2 in each measurement sample area 16 is time sequentially patterned. The time sequential pattern showing the presence/absence of the vehicle is outputted as the decided result of the vehicle presence in each measurement sample area corresponding to the moving condition of the vehicle. As the result of the decision, namely, as the feature of the pattern, the moving condition of the vehicle can be measured. Therefore, it is not necessary to follow-up the specified candidate vehicle, and the moving condition of the vehicle in the lane is grasped from the pattern. Therefore, any complicated hardware is not required for fallowing-up the candidate vehicle.