PURPOSE: To provide a weatherproof optical tester in which surface temperature of a sample is made uniform.

CONSTITUTION: A deformed C-shaped three-stage cover having inclining opposite end parts is employed in order to position samples 4 on a substantially same light orientation curve wherein each stage is provided with two irradiation windows 5 of same size. Furthermore, elongated holes 9 for fixing a sample stopper 3 are made in the frame 8. Since the sample 4 is secured while abutting, at one end part thereof, on the sample stopper 3 by means of the irradiation windows 5a, 5b fixed with the sample stopper 3, an opening 26 of predetermined width is provided between the end of sample and the end part of the irradiation window. In a weatherproof optical tester euipped with such sample holder 2a, the air flows on the surface side of the sample frown the surface side to the rear surface side of the opening 26 and the air flows uniformly over the entire surface of the sample thus making uniform the temperature over the entire surface of the sample.