A headlight optical axis adjustment method where, on an imaged picture of a light distribution pattern of low beam illumination from a master headlight whose optical axis has been adjusted, a relative positional relationship between a horizontal and oblique line of a light/dark boundary line. A reference point is determined on the imaged picture to correspond to a bulb center. An acceptance frame is set based on an imaged position of an intersection of the horizontal and oblique line. On an imaged picture of a light distribution pattern of low beam illumination from a headlight to be adjusted, a horizontal and oblique line of a light/dark boundary line are detected based on a relative positional relationship with the reference point, and the optical axis of the headlight to be adjusted is adjusted so that an intersection of the detected horizontal and oblique lines is within the acceptance frame.