PURPOSE: To provide cooling liquid sealing structure where optical performance of cooling liquid cannot be deteriorated by preventing breakage of a packing in sealing the cooling liquid, and preventing mixing of a foreign material into the cooling liquid without deteriorating a sealing effect.

CONSTITUTION: A cooling liquid injecting hole 2 formed in a housing 3 is constituted of a screw portion 2a, a non-screw portion 2b, and a step portion 2c between the portions 2a, 2b. In the step portion 2c, a packing 7 is brought into pressure-contact with the housing 3 via a washer 8 by a screw 6, and cooling liquid 1 is sealed into the housing 3.