A normal-line detection method for finding the normal vector of a measurement surface of a measurement target by means of at least one distance detector and calculating the normal vector from the measurement results thus obtained is provided. The normal vector of the measurement surface is found by calculating the exterior product of a first vector which connects a first measurement point and a second measurement point and a second vector which connects a third measurement point and a fourth measurement point and is shifted in parallel such that an end thereof is set at any one of the first measurement point and the second measurement point.
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