Provided is an apparatus for fixing a solid immersion lens (SIL) in an optical system. The apparatus includes a first adapter; a second adapter screw-coupled to the inner surface of the first adapter and screw-coupled to the outer surface of a third adapter to pressurize a first O-ring; the third adapter screw-coupled to the inner surfaces of the second adapter and a fourth adapter; the fourth adapter screw-coupled to the outer surface of the third adapter to pressurize a second O-ring; the first and second O-rings elastically fixing an outer surface of a barrel of an infrared ray (IR) objective lens; and an SIL holder mounting the SIL onto the first adapter.