To provide an interference measuring method, an interference measuring apparatus, an optical element, an optical system, and a projection aligner which enables measurement without errors, by solving the following problem, wherein measurement results have errors due to the infiltration of oxygen into an interference optical system, the strains in the interference optical system, and the fluctuations in the interference optical system caused by the strains of the containers of an interference-measuring apparatus.

This interference-measuring method performs interference measurement of an optical element or optical system to be measured by using an interference-measuring apparatus composed of a first container 15, a second container 1, which is housed in the first container and has inside it an interference optical system for performing interference measurement of the optical system or optical element, a light source 2, and a detector 8. The method has a first supply process for supplying a first gas which is different from the atmosphere into the first container, a second supply process for supplying a second gas different form the atmosphere into the second container, and an interference-measuring process for performing the interference measurement of the optical element or optical system. The interference-measuring process is carried out, after the finishing of the second supply process.