An infrared detector, comprising: a reflection portion for transmitting rays in the far-infrared range and in the middle-infrared range and for reflecting rays in the near-infrared range and in the visible range; a photo-current generating portion having a quantum well structure configured so that electrons are excited by the rays in the far-infrared range and in the middle-infrared range having passed through the reflection portion and so that the electrons thus excited generate photo-current; a light emitting portion having a quantum well structure arranged so that electrons of the photo-current generated by the photo-current generating portion are injected and in which the electrons thus injected there into are recombined with holes so as to emit rays in the near-infrared range and in the visible range; a photo-detecting portion for detecting the rays in the near-infrared range and in the visible range emitted from the light emitting portion and for detecting the rays in the near-infrared range and in the visible range emitted from the light emitting portion and reflected by the reflection portion, and a substrate; wherein said infrared detector is configured so that incident rays in the far-infrared range and in the middle-infrared range are 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 the light emitting portions includes a quantum well layer and a barrier layer as a contact layer; characterized in that the reflection portion, the photo-current generating portion, the light emitting portion and the photo-detecting portion are all made of group III-V compound semiconductors that are layered on top of the substrate made of a group III-V compound semiconductor so as to form a single united body, the photo-detecting portion is made of an avalanche photo diode and includes, in this order, an APD upper-electrode layer as a contact layer in direct contact with said barrier layer, a spacer layer, a light absorbing layer for absorbing the rays entered into the photo-detecting portion so that carriers are formed and accelerated, a spacer layer, a multiplication layer for avalanche-multiplying the carriers so that a photo-current is generated, and an APD lower-electrode layer; a single ground electrode shared by the light emitting portion and the photo-detecting portion is disposed onto the barrier layer or the APD upper-electrode layer, a negative electrode is disposed on the reflection portion and is configured to apply a negative voltage thereto, and a positive electrode is disposed on the photo-detecting portion and is configured to apply a positive voltage thereto. a reflection portion for transmitting rays in the far-infrared range and in the middle-infrared range and for reflecting rays in the near-infrared range and in the visible range; a photo-current generating portion having a quantum well structure configured so that electrons are excited by the rays in the far-infrared range and in the middle-infrared range having passed through the reflection portion and so that the electrons thus excited generate photo-current; a light emitting portion having a quantum well structure arranged so that electrons of the photo-current generated by the photo-current generating portion are injected and in which the electrons thus injected there into are recombined with holes so as to emit rays in the near-infrared range and in the visible range; a photo-detecting portion for detecting the rays in the near-infrared range and in the visible range emitted from the light emitting portion and for detecting the rays in the near-infrared range and in the visible range emitted from the light emitting portion and reflected by the reflection portion, and a substrate; wherein said infrared detector is configured so that incident rays in the far-infrared range and in the middle-infrared range are 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 the light emitting portions includes a quantum well layer and a barrier layer as a contact layer; characterized in that the reflection portion, the photo-current generating portion, the light emitting portion and the photo-detecting portion are all made of group III-V compound semiconductors that are layered on top of the substrate made of a group III-V compound semiconductor so as to form a single united body, the photo-detecting portion is made of an avalanche photo diode and includes, in this order, an APD upper-electrode layer as a contact layer in direct contact with said barrier layer, a spacer layer, a light absorbing layer for absorbing the rays entered into the photo-detecting portion so that carriers are formed and accelerated, a spacer layer, a multiplication layer for avalanche-multiplying the carriers so that a photo-current is generated, and an APD lower-electrode layer; a single ground electrode shared by the light emitting portion and the photo-detecting portion is disposed onto the barrier layer or the APD upper-electrode layer, a negative electrode is disposed on the reflection portion and is configured to apply a negative voltage thereto, and a positive electrode is disposed on the photo-detecting portion and is configured to apply a positive voltage thereto.