To provide a thermal infrared detection element which improves an effective absorption rate and can improve response speed and sensitivity.

The detection element is provided with a substrate 1, a membrane 10 formed on the main face of the substrate 1, a heat separation area 11 formed by removing a part of a semiconductor at the lower part of the membrane 10, a thermo-pile 15 formed in the membrane and a pair of reflection mirrors (total reflection mirror 21 and semi-reflection mirror 22) which face each other so that at least a part of an infrared absorption film 4 is arranged on an optical axis.