An infrared detector comprising: a reflection portion configured to transmit rays in the far-infrared range and in the middle-infrared range and to reflect rays in the near-infrared range and in the visible range; a photo-current generating portion having a plurality of layered quantum dot structures in each of which electrons can be excited by the rays in the far-infrared range and in the middle-infrared range having passed through the reflection portion and in each of which the electrons thus excited can generate photo-current; a light emitting portion having a plurality of layered quantum well structures into each of which electrons of the photo-current generated by the photo-current generating portion can be injected and in each of which the electrons thus injected there into can be recombined with holes so as to emit rays in the near-infrared range and in the visible range; and a photo-detecting portion (4, 7) configured to detect the rays in the near-infrared range and in the visible range emitted from the light emitting portion and to detect the rays in the near-infrared range and in the visible range emitted from the light emitting portion and then reflected by the reflection portion, the infrared detector wherein all of the reflection portion, the photo-current generating portion, the light emitting portion, and the photo-detecting portion are made of group III-V compound semiconductors, and are layered on the substrate made of a group III-V compound semiconductor so as to form a single united body; the reflection portion, the photo-current generating portion, the light emitting portion and the photo-detecting portion (4, 7) are arranged in this order from an incident-surface side of the infrared detector, and incident rays in the far-infrared range and in the middle-infrared range can be detected by the detection, performed by the photo-detecting portion, of the rays in the near-infrared range and in the visible range emitted from the light emitting portion; wherein in the quantum dot structure of the photo-current generating portion, a plurality of quantum dots that are designed to function as a quantum well are embedded in a barrier layer, a film thickness of a barrier layer which is included in the photo-current generating portion and which is adjacent to the light emitting portion is larger than a film thickness of the other barrier layer of the photo-current generating portion and is smaller than an electron mean free path, and a composition ratio of the barrier layer which is included in the photo-current generating portion and which is adjacent to the light emitting portion is gradually changed in the film-thickness direction so that a band gap is gradually narrowed down towards the light emitting portion. a reflection portion configured to transmit rays in the far-infrared range and in the middle-infrared range and to reflect rays in the near-infrared range and in the visible range; a photo-current generating portion having a plurality of layered quantum dot structures in each of which electrons can be excited by the rays in the far-infrared range and in the middle-infrared range having passed through the reflection portion and in each of which the electrons thus excited can generate photo-current; a light emitting portion having a plurality of layered quantum well structures into each of which electrons of the photo-current generated by the photo-current generating portion can be injected and in each of which the electrons thus injected there into can be recombined with holes so as to emit rays in the near-infrared range and in the visible range; and a photo-detecting portion (4, 7) configured to detect the rays in the near-infrared range and in the visible range emitted from the light emitting portion and to detect the rays in the near-infrared range and in the visible range emitted from the light emitting portion and then reflected by the reflection portion, the infrared detector wherein all of the reflection portion, the photo-current generating portion, the light emitting portion, and the photo-detecting portion are made of group III-V compound semiconductors, and are layered on the substrate made of a group III-V compound semiconductor so as to form a single united body; the reflection portion, the photo-current generating portion, the light emitting portion and the photo-detecting portion (4, 7) are arranged in this order from an incident-surface side of the infrared detector, and incident rays in the far-infrared range and in the middle-infrared range can be detected by the detection, performed by the photo-detecting portion, of the rays in the near-infrared range and in the visible range emitted from the light emitting portion; wherein in the quantum dot structure of the photo-current generating portion, a plurality of quantum dots that are designed to function as a quantum well are embedded in a barrier layer, a film thickness of a barrier layer which is included in the photo-current generating portion and which is adjacent to the light emitting portion is larger than a film thickness of the other barrier layer of the photo-current generating portion and is smaller than an electron mean free path, and a composition ratio of the barrier layer which is included in the photo-current generating portion and which is adjacent to the light emitting portion is gradually changed in the film-thickness direction so that a band gap is gradually narrowed down towards the light emitting portion.