A cryostat includes: a casing having an inlet port and exit port
a cell housing provided in the casing
a temperature controller for adjusting the temperature of the cell
a first optical path tube for guiding a light beam from the inlet port of the casing to the cell housing
a second optical path tube for guiding the light beam having passed through the cell housing to the exit port of the casing
first and second optical windows disposed at openings, exposed to the outside, of the first and second optical path tubes, respectively
and sealing materials having a water vapor transmission rate of 30000 cccm2mmseccm Hg1010 or lower, disposed at the peripheries of the first and second optical windows to seal the first and second optical path tubes.