Spectroscopic method and spectrometer

A light absorption coefficient of the sample to achieve a device for measuring with high sensitivity. A ring-down spectroscopy apparatus using the wavelength-tunable femtosecond soliton pulse light source 1. Pulsed light, first optical transmission path 4 through the optical changeover switch 5 is inputted to the looped optical fiber 6. Ring-down pulse light through the optical changeover switch 5, and inputs to the homodyne detector 40. On the other hand, the pulse light propagating through the first optical transmission path 4, the optical directional coupler 8, via the first optical switching element 12 is branched inputted to the optical transmission line constituting a second optical transmission path 20 . Pulse light propagating through the second optical transmission line 20 as reference light, it is input to the homodyne detector 40 is a synchronous detection. Optical path lengths of the plurality of optical transmission lines constituting the second optical transmission line 20, the length of the optical fiber 6 by sequentially with different, each of the optical transmission path, the optical path length can be small fluctuations.