PURPOSE: To facilitate the automation of the detection of the direction, by using the distance between the image of a marker by a lens on an image sensor and the optical axis of the lens, and obtaining the angle of the optical axis with respect to a base line connecting the marker and the lens by a specified formula.

CONSTITUTION: The advancing directions of the working of various working vehicles are to be acknowledged. For example, a pair of cameras 5A and 5B is arranged so that the cameras are placed at an equal distance with respect to a central line la of a turntable 2, on which excavating apparatus 3 is provided. A marker 8 is provided on an imaginary running objective line lc. The cameras 5A and 5B, each comprising an image sensor 6 and a lens 7, are fixed so that the optical axis ld of the lens is in parallel with the line la. An image 8a of the marker 8 is formed on the sensor 6 of each of the cameras 5A and 5B. The displacement distance (d) with respect to the axis ld is obtained. The angle θ, which is formed by the base line lx connecting the marker 8 and the lens 7 and the axis ld is obtained by θ=arctan d/l based on the distance l between the lens 7 and the sensor 6 and the distance (d).