Abstract: That illuminating pattern of the headlight which appears on a screen disposed in front of the vehicle is pictured. The optical axis of the headlight is adjusted based on an image of the illuminating pattern. A distribution of illuminance of the illuminating pattern along a scanning line which bridges a light portion and a dark portion of the image of the illuminating pattern is measured. A position of a reference point which serves as a reference in adjusting the optical axis is obtained from that point on the scanning line which corresponds to a crossing point of a first straight line and a second straight line. The first straight line is obtained from a portion corresponding at least to one of the light portion and the dark portion of a curve which represents the measured distribution of illuminance. The second straight line is obtained from a portion corresponding to a transient region which is positioned between the light portion and the dark portion.