Provided is an optical densitometer for measuring a density of a gas or liquid of interest, the optical densitometer comprising: a light source capable of introducing light into a core layer; a detector capable of receiving the light that has propagated through the core layer; and an optical waveguide, the optical waveguide comprising: a substrate; and the core layer comprising a light propagation portion capable of propagating the light in an extending direction of the light propagation portion, and a diffraction grating portion, the diffraction grating portion comprising a diffraction grating region and an extension region connected to the diffraction grating region, and a first optical coupling region included in the extension region and a second optical coupling region included in the light propagation portion being optically coupled with respect to the light propagating through the core layer.