PURPOSE: To achieve an evaluation of a detection element without effect of background radiation by a method wherein a light shielding plate is arranged to cover all except for a light shielding tube located between an infrared-ray source and a cold stage and a shutter plate and a rotary blade of a chopper are made as high reflection surfaces.

CONSTITUTION: A light shielding plate 2 has a hole the same in dimension as an opening of an infrared-ray source 7 at the center thereof an located on the side of a cold stage 8 from a shutter plate 5 to be fixed on the infrared-ray source 7. Both surfaces of the plate is made high in reflectivity. A light shielding tube 1 is cylindrical with an inner diameter larger than a window 15 and provided between the light shielding plate 2 and a top plate 14, the surface of an internal wall thereof is made high in reflectivity. Both surfaces of the shutter plate 5 and a rotary blade 3 of a chopper are made high in reflectivity. With such an arrangement, irradiation of infrared rays can be realized at an optional opening under no complete darkness and background radiation thereby enabling the evaluating of an infrared detection element in a pellet or in a wafer without effect of the background radiation.