PURPOSE: To suppress a temperature rise in an optical dome due to heat generated at the time of flying at a high speed, to prevent deterioration of performance of the dome and to simultaneously increase a flying speed range.

CONSTITUTION: A high pressure gas source 4 is mounted in a body, and a high pressure tube 5 is connected. A gas discharge hole 6 provided on a central axis of an optical dome 2 is connected to the end of the tube 5, gas from the source 4 is guided to the hole 6 through the tube 5, and is sprayed to the inner surface of the dome 2. A gas discharger 7 is provided in the body to discharge the gas discharged into the dome 2 out of the dome 2.