PURPOSE: To make it possible to measure the dark output characteristics or the optical characteristics of an infrared detection element without replacing the cold shield by providing an opening/closing shade at an opening of the cold shield through which infrared rays pass.

CONSTITUTION: A light entering window 40 made of silicon is provided at a part of an outer tube 2. An infrared detection element is fixed to a cooling block and a cold shield 60 is disposed around the infrared detection element. An opening 61 is provided so that an infrared rays 10 entering through the window 40 arrives at the infrared detection element and an opening/closing shade 62 is provided for the cold shield 60 while covering the opening 61. The shade 62 can be opened/closed externally by magnetic force, light, gravity, etc. Since the cold shield 60 is not required to be replaced, vacuum can be maintained. This vessel significantly shortens the time required for measurement of the characteristics of infrared element or astronomical measurement.