A spectrometer comprising: a stacked light absorbing structure comprising a plurality of absorbing layers (111, 112, 113) stacked in a vertical direction and having a plurality of absorption wavelength bands, and a plurality of tunnel junction layers respectively interposed between the plurality of absorbing layers to electrically connect the plurality of absorbing layers; an illuminating unit configured to select an absorbing layer from among the plurality of absorbing layers, the selected absorbing layer being of one absorption wavelength band of the plurality of absorption wavelength bands, and to provide the stacked light absorbing structure with an illumination light including other absorption wavelength bands of the plurality of absorption wavelength bands of other absorbing layers of the plurality of absorbing layers; wherein the plurality of absorbing layers (111, 112, 113) are arranged in descending order of energy band gap from a light input side facing the illumination unit, characterised in that: each of the plurality of tunnel junction layers has a larger energy band gap than the plurality of absorbing layers; the illuminating unit comprises: a light source configured to generate an illumination light including the plurality of absorption wavelength bands of the plurality of absorbing layers; and a notch filter array disposed between the light source and the stacked light absorbing structure and comprising a plurality of notch filters (132a, 132b, 132c, 132d) having a plurality of absorption characteristics to block only an illumination light of the absorption wavelength band of the selected absorbing layer from among the illumination light generated by the light source and transmit an illumination light of the other absorption wavelength bands; and the spectrometer further comprises a controller configured to control the notch filter array to rotate to change sequentially the blocked absorption wavelength band among the plurality of absorption wavelength bands with the lapse of time. a stacked light absorbing structure comprising a plurality of absorbing layers (111, 112, 113) stacked in a vertical direction and having a plurality of absorption wavelength bands, and a plurality of tunnel junction layers respectively interposed between the plurality of absorbing layers to electrically connect the plurality of absorbing layers; an illuminating unit configured to select an absorbing layer from among the plurality of absorbing layers, the selected absorbing layer being of one absorption wavelength band of the plurality of absorption wavelength bands, and to provide the stacked light absorbing structure with an illumination light including other absorption wavelength bands of the plurality of absorption wavelength bands of other absorbing layers of the plurality of absorbing layers; wherein the plurality of absorbing layers (111, 112, 113) are arranged in descending order of energy band gap from a light input side facing the illumination unit, characterised in that: each of the plurality of tunnel junction layers has a larger energy band gap than the plurality of absorbing layers; the illuminating unit comprises: a light source configured to generate an illumination light including the plurality of absorption wavelength bands of the plurality of absorbing layers; and a notch filter array disposed between the light source and the stacked light absorbing structure and comprising a plurality of notch filters (132a, 132b, 132c, 132d) having a plurality of absorption characteristics to block only an illumination light of the absorption wavelength band of the selected absorbing layer from among the illumination light generated by the light source and transmit an illumination light of the other absorption wavelength bands; and the spectrometer further comprises a controller configured to control the notch filter array to rotate to change sequentially the blocked absorption wavelength band among the plurality of absorption wavelength bands with the lapse of time. each of the plurality of tunnel junction layers has a larger energy band gap than the plurality of absorbing layers; the illuminating unit comprises: a light source configured to generate an illumination light including the plurality of absorption wavelength bands of the plurality of absorbing layers; and a notch filter array disposed between the light source and the stacked light absorbing structure and comprising a plurality of notch filters (132a, 132b, 132c, 132d) having a plurality of absorption characteristics to block only an illumination light of the absorption wavelength band of the selected absorbing layer from among the illumination light generated by the light source and transmit an illumination light of the other absorption wavelength bands; a light source configured to generate an illumination light including the plurality of absorption wavelength bands of the plurality of absorbing layers; and a notch filter array disposed between the light source and the stacked light absorbing structure and comprising a plurality of notch filters (132a, 132b, 132c, 132d) having a plurality of absorption characteristics to block only an illumination light of the absorption wavelength band of the selected absorbing layer from among the illumination light generated by the light source and transmit an illumination light of the other absorption wavelength bands; and the spectrometer further comprises a controller configured to control the notch filter array to rotate to change sequentially the blocked absorption wavelength band among the plurality of absorption wavelength bands with the lapse of time.