A robot apparatus 1 is to be electrically charged autonomously. An electrical
charging device 100 is provided with two markers, namely a main marker 118
and a sub-marker 119, and the heights of the markers are pre-stored in the
robot apparatus. When the robot apparatus 1 is to find the direction and
the distance to the electrical charging device 100, a CCD camera 20
finds the direction vector of the marker from the photographed image. This direction
vector is transformed into a position vector of a camera coordinate system {c}
and further into a position vector of the robot coordinate system {b}. The coordinate
in the height-wise direction in the robot coordinate system {b} is compared to
the pre-stored height to find the distance between the markers and the robot apparatus
and the direction of the robot apparatus.