laser beam spot 8 in this case the measurable limit of the outer diameter of the pin 6 is almost the same as that of the diameter of the beam projecting and condensing convex lenses 3 and 4 the output of the arithmetic processing unit 10 is applied to the display unit 11 such as a cathode ray tube to display the outer diameter of the first pin 6 on the display unit 11 the arithmetic processing unit 10 may be an electronic computer or any other suitable arithmetic circuit in the first embodiment described above the first pin 6 whose outer diameter is unknown is disposed closely adjacent to the second pin 7 whose outer diameter has been previously accurately measured another embodiment which is a modification of the first embodiment is shown in fig5 a and 5 b in which the first wire rod on pin 6 and the second wire rod on pin 7 are spaced apart from each other in the advancing direction x of the laser beam spot in fig5 a these pins 6 and 7 are spaced apart by a center to center distance l from each other in such a pin arrangement the unknown outer diameter d of the first pin 6 is measured on the basis of the known outer diameter d of the second pin 7 by a method which will be described below first as shown in fig5 a the first pin 6 and the second pin 7 are disposed in a relation in which they partly overlap with each other in the scanning direction s substantially orthogonal with respect to the advancing direction x of the laser beam spot and a dimension a in such a pin arrangement is measured this.