In a space navigation optical instrument comprising a barrel for housing therein an optical part and a contaminant removing door, provided at a part of the outer periphery of the barrel which has the largest radiant heat coupled to the space during a space navigation and has the least external heat input, for opening the inside of the barrel to the space, a thermal gradient is generated in which the inside of the barrel is of a high temperature and the space is of a low temperature, and the inside of the barrel is opened to the space by opening the contaminant removing door to discharge contaminants into the space.