PURPOSE: To provide a measuring method and its apparatus which are hardly subjected to the influence of a surrounding environment and in which an optical path can be changed easily.

CONSTITUTION: An object 10, to be measured, inside a chamber 12 is irradiated with coherent light, and the physical quantity of the object 10 to be measured is measured. There are installed irradiation means 16, 20 which irradiate the object 10 to be measured and measuring means 22, 26, 28 which measure the physical quantity on the basis of the interference light intensity of the transmitted light or the reflected light of the object 10 to be measured. A measuring optical-path member 30 which is composed of a substance, through which the coherent light is transmitted, and which form a part of the optical path of the irradiated light from the irradiation means 16, 20, of the transmitted light or of the reflected light of the object 10 to be measured is inserted into the chamber 12 in which the object 10 to be measured is placed.