Abstract: Apparatus is disclosed for sensitively detecting and quantitatively analyzing biological and biochemical substances. Charged into a cell is a test molecule labeled with a fluorescent dyestuff. An exciting optical system transmits plane polarized light from a light source into the labeled molecule to excite the molecule, and a polarizing optical system takes up fluorescent light emitted from the molecule. A photomultiplier is arranged to measure the fluorescence intensities of parallel and perpendicularly polarized light from the molecule. A calculator coordinated with the polarizing optical system computes and converts the fluorescence intensities into the degree of polarization of the molecule.