Normal detection method, normal detection device, and machining machine provided with normal detection function

A normal detection method for measuring the distance to a measurement subject using one or a plurality of distance detectors, and obtaining a normal vector on the measured surface of the measurement subject from the obtained measurement result, wherein: within a three-dimensional space, the straight line linking a first measurement point measured at a first measurement position using a distance detector and a second measurement point measured at a second measurement position different from the first measurement position is set as a first vector; the straight line linking 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 as a second vector; and a normal vector on the measured surface is obtained by determining the vector product of the first vector and the second vector.