Abstract: An infrared ray detecting sensor having an infrared ray detecting element located in the cool chamber of a Dewar structure comprising inner and outer cylindrical members. The chamber is evacuated to vacuum level and a getter is arranged in the chamber for ensuring the vacuum level in the chamber. A partition is arranged in the chamber between the inner end of the inner cylindrical member and the getter for forming a restricted passage and allowing communication of a gas between the getter and the infrared ray detecting element and obstructing a stray ray generated upon a radiation of the getter toward the infrared ray detecting element. The restricted passage is formed such that a ray generated upon radiation of the getter is reflected many times at the restricted passage and is dampened. The partition is roughened and colored in black.