To provide a stable optical axis adjustment method of a headlight acquired by improving a technology developed already, capable of coping with an actual situation of a present optical axis adjustment method of the head lamp, and having higher accuracy.

This method is characterized as follows: a horizontal line is determined from a balance position of a horizontal balancer and an inclined line is determined at a designated angle from a balance position of an inclined sensor as shown in figure 4 by a sensor system (figure 3) for performing optical axis adjustment so that three-spot sensors on designated coordinates are balanced; the intersection point thereof is used as a virtual elbow point, and the position of the virtual elbow point is defined as '0'; a light and shade boundary point is determined at a prescribed interval in the vertical direction by a set value of inclination of the light and shade boundary point; therefore an inclined cut line is determined in a range at a prescribed interval and a horizontal cut line is determined at a prescribed interval by a method for determining the light and shade boundary point (cut line) by an approximate expression (least-squares method); and an intersection point combined therefrom is determined to be an elbow point, and optical axis adjustment is performed so that the determined elbow point falls within a specification range.