Abstract: Disclosed is an apparatus for observing the interior of a hot furnace. The apparatus is equipped with a camera arranged in a case whose inner surface is cooled by means of cooling water and into whose inner space cooling air is blown, the apparatus comprising: a quartz-glass plate, a heat-ray-reflecting-glass plate, a heat-ray-absorbing-glass plate, and a heat-ray-transmitting mirror arranged in order of decreasing distance to the camera in an optical path through which a ray of light entering at the front surface of the case is led to the camera; and an illuminating lamp which is provided on the front surface of the case and whose luminance can be remote-controlled. With this construction, the temperature in the camera case can be kept at 50.degree. C. or less, and damage to the camera due to the radiation heat entering through the glass window can be avoided. With this apparatus, the interior of a hot furnace can be observed over a temperature range of room temperature to 1200.degree. C.