Disclosed are a disturbance light identifying apparatus, a disturbance light separating apparatus, a disturbance light identifying method, and a disturbance light separating method capable of precisely identifying whether or not light exiting an optical system contains a disturbance light component or capable of separating such a disturbance light component by using a simple technique. Provided are: a modulated light irradiation unit that irradiates an optical system 1 with modulated light; a light receiving unit that receives light exiting the optical system 1 in response to an incidence of the modulated light from the modulated light irradiation unit; and a controlling unit that controls the modulated light irradiation unit and the light receiving unit.