An ellipsometer includes a light source for generating a probe beam of polychromatic light for interacting with a sample. Each distinct lighting source is comprised of one of the LED based lighting elements and a given pair of the end caps, whereby distinct light sources can be formed through replacement of the LED based lighting elements and the end caps. Accordingly, the increase of compression work of the compressor is prevented, and an opening area of an oil return hole can be expanded to such a degree that an optimum amount of lubricating oil is attained at the time of the heating operation. Unreliability of each item in the failure-event tree is calculated based on a failure unreliability function.