The imaging position of each of the frames in image data of a plurality of frames captured while a vehicle is traveling is accurately determined. An image data acquiring device captures a front image by means of a video camera while a vehicle is traveling. When in imaging, the device associates the vehicle speed pulse detected by a vehicle speed sensor with the frame data and records them. An image data processing device arranges data on each frame of the image along the initial path according to the correspondence with the vehicle speed pulse. The device determines the variation between the frames of a feature point such as a road lane marking included in the image, reflects the variation on the initial path, and corrects the errors in the direction perpendicular to the moving direction so as to determine the traveling path and imaging positions of the frames.