PURPOSE: To facilitate adjustment through a simple structure, by detecting a position of a moving mirror of an interference meter and center position of the moving mirror by the use of a laser light flux and a white light flux.

CONSTITUTION: Light from a white light source S3 produces a parallel light flux ω by a lens, the light flux ω and a light flux emitted from a laser source S2 are parallel to each other, and are adjusted so that they are brought to a parallel to a measuring infrared light flux entering a translucent mirror BS from a collomater CM. 3 types of light fluxes enter the translucent mirror BS, a part of them reflect, a part of them transmit, they independently interfer, and are radiated from an interference meter. Laser rays emitted from the interference meter are reflected in a direction of a samle chamber SC without a change in the direction thereof. In order to detect a center position of a moving mirror Mm, a position, where an output of a photo detector D2 is maximized, is detected by lighting a white light source S3 and by moving mirror Mm. The position of the moving mirror Mm is detected by detecting laser rays by a photo detector D3.