A normal-line detection method for finding a normal vector (Vn) of a measurement surface of a measurement target by measuring a plurality of distances (La, Lb, Lc, Ld, Le, Lf, Lg, Lh) to the measurement target by means of at least one distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) and calculating the normal vector (Vn) from measurement results (La, Lb, Lc, Ld, Le, Lf, Lg, Lh) thus obtained, the normal-line detection method characterized by comprising the steps of: expressing, in three-dimensional coordinates, a plurality of measurement points (Qa, Qb, Qc, Qd, Qe, Qf, Qg, Qh) on the measurement surface corresponding to a plurality of measurement positions (Pa, Pb, Pc, Pd, Pe, Pf, Pg, Ph) from which the distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) measures the distances (La, Lb, Lc, Ld, Le, Lf, Lg, Lh) to the measurement target, on the basis of the plurality of measurement positions (Pa, Pb, Pc, Pd, Pe, Pf, Pg, Ph) and the plurality of measurement results (La, Lb, Lc, Ld, Le, Lf, Lg, Lh) obtained by the distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) from the plurality of measurement positions (Pa, Pb, Pc, Pd, Pe, Pf, Pg, Ph); setting, as a first vector (Vae), a straight line connecting a first measurement point (Qa) and a second measurement point (Qe) in a three-dimensional coordinate system, the first measurement point (Qa) being a point on the measurement target a distance to which is measured by the distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) from a first measurement position (Pa) being any one of the plurality of measurement positions (Pa, Pb, Pc, Pd, Pe, Pf, Pg, Ph), the second measurement point (Qe) being a point on the measurement target a distance to which is measured by the distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) from a second measurement position (Pe) located opposite to the first measurement position (Pa); setting, as a second vector (V'cg), a straight line connecting a third measurement point (Qc) and a fourth measurement point (Qg) in the three-dimensional coordinate system and shifted in parallel such that an end thereof is set at any one of the first measurement point (Qa) and the second measurement point (Qe), the third measurement point (Qc) being a point on the measurement target a distance to which is measured by the distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) from a third measurement position (Pc) located between the first measurement position (Pa) and the second measurement position (Pe), the fourth measurement point (Qg) being a point on the measurement target a distance to which is measured by the distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) from a fourth measurement position (Pg) located between the first measurement position (Pa) and the second measurement position (Pe) and being opposite to the third measurement position (Pc); and finding the normal vector (Vn) of the measurement surface by calculating an exterior product of the first vector (Vae) and the second vector (V'cg). expressing, in three-dimensional coordinates, a plurality of measurement points (Qa, Qb, Qc, Qd, Qe, Qf, Qg, Qh) on the measurement surface corresponding to a plurality of measurement positions (Pa, Pb, Pc, Pd, Pe, Pf, Pg, Ph) from which the distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) measures the distances (La, Lb, Lc, Ld, Le, Lf, Lg, Lh) to the measurement target, on the basis of the plurality of measurement positions (Pa, Pb, Pc, Pd, Pe, Pf, Pg, Ph) and the plurality of measurement results (La, Lb, Lc, Ld, Le, Lf, Lg, Lh) obtained by the distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) from the plurality of measurement positions (Pa, Pb, Pc, Pd, Pe, Pf, Pg, Ph); setting, as a first vector (Vae), a straight line connecting a first measurement point (Qa) and a second measurement point (Qe) in a three-dimensional coordinate system, the first measurement point (Qa) being a point on the measurement target a distance to which is measured by the distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) from a first measurement position (Pa) being any one of the plurality of measurement positions (Pa, Pb, Pc, Pd, Pe, Pf, Pg, Ph), the second measurement point (Qe) being a point on the measurement target a distance to which is measured by the distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) from a second measurement position (Pe) located opposite to the first measurement position (Pa); setting, as a second vector (V'cg), a straight line connecting a third measurement point (Qc) and a fourth measurement point (Qg) in the three-dimensional coordinate system and shifted in parallel such that an end thereof is set at any one of the first measurement point (Qa) and the second measurement point (Qe), the third measurement point (Qc) being a point on the measurement target a distance to which is measured by the distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) from a third measurement position (Pc) located between the first measurement position (Pa) and the second measurement position (Pe), the fourth measurement point (Qg) being a point on the measurement target a distance to which is measured by the distance detector (30a, 30b, 30c, 30d, 30e, 30f, 30g, 30h) from a fourth measurement position (Pg) located between the first measurement position (Pa) and the second measurement position (Pe) and being opposite to the third measurement position (Pc); and finding the normal vector (Vn) of the measurement surface by calculating an exterior product of the first vector (Vae) and the second vector (V'cg).