This invention is related to an optical-analysis-type raw material fluid density detector including a detector main body and a light oscillation unit and a light detection unit that are provided on the upper surface or the under surface of the detector main body, in which the detector main body has at least one recess formed in the upper surface and the under surface, a fluid flow path connecting a fluid inlet of the detector main body to the recess, a fluid flow path connecting the recesses to each other, and a fluid flow path connecting the recess to a fluid outlet of the detector main body; the light oscillation unit is disposed in the recess that is closest to the inlet; and light detection units are disposed in the remaining recesses.