To provide a method capable of accurately and simply detecting the center (elbow point) of a light distribution pattern having a horizontal cut line and an oblique cut line.
An imaging device 113 with the first light receiving domain A1 and the second light receiving domain A2 arranged oppositely in the vertical direction across an upper-lower middle domain A3 is used, and the light distribution pattern LBP is received, while changing the position of the imaging device 113 in the vertical direction, and a middle point in a range when a light receiving ratio between each light receiving amount detected in the first light receiving domain A1 and the second light receiving domain A2 becomes an extreme value is detected as a position in the vertical direction of the horizontal cut line HC. Even when a wavy part HW caused by cut dimness is generated on the horizontal cut line HC, the wavy part HW is removed, and the horizontal cut line HC and the elbow point EP positioned above it can be detected with high accuracy.
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