PURPOSE: To enable clear and concentrated observation of a part of a sample which is hard to observe by an inner-tank light alone, in a constant-temperature constant-humidity tank.

CONSTITUTION: A spotlight 5 is so provided inside an opening-closing door 1 as to be directed toward the interior of a tank and the rear part 22 of the spotlight 5 is fitted to a second rotating shaft 12. While this second rotating shaft 12 is rotated in normal and reverse directions by a second motor 11, it is provided, at the same time, on a first rotating shaft 10 through the intermediary of a second bearing 16 so that it intersects the first rotating shaft 10 perpendicularly, and it can be rotated in the axial direction as the whole of the shaft by a first motor 9. By appropriate rotation of the first motor 9 and the second motor 11, therefore, the light of the spotlight 5 can be applied to an arbitrary place inside the tank.