PURPOSE: To enable to perform an automatic and self-measurement of a pore diameter, by a method wherein a measuring surface is scanned by semiconductor laser spot light to detect an irradiation light strength of semiconductor laser varying with reflection light.

CONSTITUTION: A measuring surface 11, wherein a pore 12 is formed, is scanned by laser light from a laser measuring device 1 placed on a table which moves in 2 directions crossing each other at right angles. A part, except the pore, reflects laser spot light, and the reflection light enters an active layer 3 of a semiconductor laser 2 after travelling backward in the same optical path. This reduces the threshold of the laser 2 and increases a radiation light strength. The increase is monitored by a photo detector 9, and based on a position signal generated with the movement of the table, an automatic and high-precise measurement is made on the diameter and position of the pore which an air micrometer can not measure at a sufficient precision.