The present invention provides a method for detecting an edge of an object by using a laser ranging device. An n number of measurement points P(i), P(i−1) and P(i+1) corresponding to an n number of projection lines that are close to each other are extracted from a plurality of measurement points, the distances of which have been measured by a laser ranging device 1, and an approximate straight line L(i) that passes through the n number of measurement points is calculated. The degree of inclination e(i) of the approximate straight line L(i) for a representative line of projection lines corresponding to the n number of measurement points is determined. If the degree of inclination is smaller than a predetermined threshold value, then one of the n number of measurement points is identified as a measurement point of an edge of an object.