1. A normal detection method for measuring a plurality of distances to a measurement subject using one or a plurality of distance detectors, and obtaining a normal vector on a measured surface of the measurement subject from the obtained measurement results, wherein a plurality of measurement points on the measured surface at a plurality of measurement positions are represented with three-dimensional coordinates from the plurality of measurement positions where the distance detectors measure the distances to the measurement subject, and a plurality of measurement results obtained by the distance detectors at the plurality of measurement positions, wherein a straight line connecting, on three-dimensional axes, a first measurement point measured at an arbitrary first measurement position among the plurality of measurement positions by the distance detector and a second measurement point measured at a second measurement position different from the first measurement position is defined as a first vector, wherein a straight line connecting, on three-dimensional axes, the first measurement point and a third measurement point measured at a third measurement position different from the first measurement position and the second measurement position is defined as a second vector, and wherein a normal vector on the measured surface is obtained by determining an outer product of the first vector and the second vector.