To provide a crystal holding device which allows the stable use of nonlinear optical crystals, such as CLBO and BBO, having a deliquescence property for a long period of time.

A holder body 2 on which a heater 5 is wound and a nonlinear optical crystal 1 in a heater bobbin 3 are installed and are housed into a housing 10. Also, extension flanges 8a and 8b having window members 13a and 13b are mounted at the housing 10. The extension flanges 8a and 8b are provided with gas introducing paths and gas discharge paths so that the gaseous oxygen or gaseous mixture described above is supplied from outside into the space installed with the nonlinear optical crystal. Also, when the gas is introduced at all times from outside, both sides of the extension flanges 8a and 8b may be opened without providing the flanges with the window members 13a and 13b. The incident surface and exit surface of the nonlinear optical crystal 1 come into contact with the gas but the flanks thereof are held airtight.