Abstract: A density meter which is immersed in a liquid to be measured to optically measure the degree of opacity thereof is disclosed. The meter comprises a density measurement system in which the liquid is sucked into a transparent inner cylinder by moving a piston upwardly and the transmittance of the liquid is measured by an optical system provided outside the transparent inner cylinder, and a calibration system in which a calibration liquid flow path is formed. The outlet of the calibration liquid flow path is communicated with the transparent inner cylinder, and the outlet is above a position at which the end of the piston is set during density measurement. In calibration, the piston is raised to a position higher than the position of the outlet to introduce a calibration liquid into the inner cylinder to calibrate the density meter by the optical system.