A spectroscopy method, includes guiding pulse laser light to an optical fiber, which mutually reacts with a sample to be measured of a light absorptance characteristic, outputting ring down pulse light obtained through light absorption of the sample, measuring an absorptance characteristic of the sample based on an attenuation characteristic of the ring down pulse light, and setting the pulse laser light as wide-spectrum laser light, setting the optical fiber as a strong dispersive optical fiber, and increasing a pulse width of the ring down pulse light to measure a wavelength absorptance characteristic based on a ring down attenuation constant of a pulse train with respect to a time sequence corresponding to a wavelength.