A spectroscopic analysis device, comprising: a film configured to generate surface plasmons and arranged to contact a sample subject to spectroscopic analysis; organic material attached to a boundary surface of the film, the boundary surface being configured to contact the sample; a first irradiator configured to irradiate a first irradiation light having a transition energy to decompose the organic material; and an optical waveguide joined to the film on the opposite side of the sample and configured to transmit the first irradiation light, wherein the optical waveguide is arranged to generate a first evanescent wave, based on the first irradiation light, on a surface (2a, 2c) of the optical waveguide, and then project the same on a region of the attached organic material, wherein the wavelength band of the first irradiation light is included in the ultraviolet range; the spectroscopic analysis device further comprising: a first detector configured to detect a first measured light emitted from the optical waveguide, wherein the first measured light is based on the first irradiation light, and the first measured light comprises a first information of an absorption spectrum of the attached organic material; and a controller configured to: obtain, from the first detector, the first information, determine, based on the obtained first information, whether a peak of the absorption spectrum has reached a threshold value relating to spectrum intensity, and when it is determined that the threshold value has been reached, change irradiation intensity of the first irradiation light using the first irradiator. a film configured to generate surface plasmons and arranged to contact a sample subject to spectroscopic analysis; organic material attached to a boundary surface of the film, the boundary surface being configured to contact the sample; a first irradiator configured to irradiate a first irradiation light having a transition energy to decompose the organic material; and an optical waveguide joined to the film on the opposite side of the sample and configured to transmit the first irradiation light, wherein the optical waveguide is arranged to generate a first evanescent wave, based on the first irradiation light, on a surface (2a, 2c) of the optical waveguide, and then project the same on a region of the attached organic material, wherein the wavelength band of the first irradiation light is included in the ultraviolet range; the spectroscopic analysis device further comprising: a first detector configured to detect a first measured light emitted from the optical waveguide, wherein the first measured light is based on the first irradiation light, and the first measured light comprises a first information of an absorption spectrum of the attached organic material; and a controller configured to: obtain, from the first detector, the first information, determine, based on the obtained first information, whether a peak of the absorption spectrum has reached a threshold value relating to spectrum intensity, and when it is determined that the threshold value has been reached, change irradiation intensity of the first irradiation light using the first irradiator. a first detector configured to detect a first measured light emitted from the optical waveguide, wherein the first measured light is based on the first irradiation light, and the first measured light comprises a first information of an absorption spectrum of the attached organic material; and a controller configured to: obtain, from the first detector, the first information, determine, based on the obtained first information, whether a peak of the absorption spectrum has reached a threshold value relating to spectrum intensity, and when it is determined that the threshold value has been reached, change irradiation intensity of the first irradiation light using the first irradiator. obtain, from the first detector, the first information, determine, based on the obtained first information, whether a peak of the absorption spectrum has reached a threshold value relating to spectrum intensity, and when it is determined that the threshold value has been reached, change irradiation intensity of the first irradiation light using the first irradiator.