Recent instruments for the celestial far-infrared observations are reviewed with some examples. They are loaded on airplanes, balloons, rockets, and satellites because of the atmospheric absorption and emission. Furthermore, some of these are cooled down below 10 degree K by liquid helium cryogen to be free from the thermal radiation noise generated by warm instruments, for example, mirrors, filters. IRAS, which is a cooled and satellite-borne telescope, had been able to detect the extremely weak far-infrared radiation from many celestial objects. It was demonstrated by IRAS that the cryogenic technique is indispensable for the far-infrared observation of high sensitivity.
