Provided are an optical device capable of effectively preventing contamination and a method for preventing contamination of the same. An optical device according to an embodiment includes a light source that generates light containing EUV (Extreme UltraViolet) light or VUV (Vacuum UltraViolet) light, a chamber in which an object to be irradiated with the light is placed, an optical element placed inside the chamber to guide the light, an introduction unit that introduces hydrogen or helium into the chamber, a power supply that applies a negative voltage to the optical element in the chamber, an ammeter that measures an ion current flowing through the optical element, and a control unit that adjusts the amount of the hydrogen or the helium introduced according to a measurement result of the ammeter.