To achieve an apparatus capable of measuring a light absorption coefficient f a sample with high sensitivity. A ring down spectroscope uses a wavelength-variable femtosecond soliton pulse light source 1. Pulse light is input to a loop optical fiber 6 through a first light waveguide 4 and a wavelength selective switch 5. Ring down pulse light is input to a homodyne detector through the wavelength selective switch 5. On the other hand, pulse light propagating in the first light waveguide 4 is split and input to light waveguides constituting a second light waveguide 20 through an optical directional coupler 8 and a first optical switching element 12. The pulse light propagating in the second light waveguide 20 is input to the homodyne detector as reference light and used for synchronous detection. The plural light waveguides constituting the second light waveguide 20 differ in optical length in accordance with the length of the optical fiber 6, and can slightly change the optical length.