PURPOSE: To prevent the generation of fume or the like due to a carbon arc and the generation of mist due to a spray, by humidifying a test piece in a water tank with circulated immersion thereof through the tank while an ultraviolet lamp is used as light source.

CONSTITUTION: A test piece support 4 is supported rotatably about a horizontal shaft 3 with a shaft on a water tank 2 with an upper opening in such a manner that a rotary circumferential sphere 4A can be immersed into the water tank 2. An ultraviolet lamp 5 for irradiation of a test piece B is provided on the top of the test piece support 4 and all of the water tank 2, the test piece support 3, the ultraviolet lamp 5 and the like are arranged entirely in a closable casing 6. A test piece 4B such as building material is stuck on the test piece support 4 is driven to rotate with a driver being subjected to an underwater immersion in the water tank 2. Thus, the test piece 4B undergoes an alternate repetition of humidification and irradiation without generating mist, thereby performing an accelerated weather test.