PURPOSE: To measure the density and water level at the same tme, by providing an elastic member having the mirror surface on one side to a case wherein the sludge enters freely and oscillating the elastic member by an oscillation means and on one hand, irradiating the light from a projector to the mirror and photodetecting its reflected light.

CONSTITUTION: An elastic member 4 having a mirror surface 4a on one side is provided to a case 3 wherein the sludge 2 enters freely and the member 4 is oscillated by an oscillation means 5. Further, light from a projector 6a is irradiated to the surface 4a and its reflected light is received by a photodetector 6b to convert it into an electric signal by a signal processing system 7. Then, the water level of the sludge is measured by detecting an oscillation frequency of the member 4 as the frequency of a detected signal and the density of the sludge is measured by detecting the photodetecting intensity of the reflected light as the amplitude of the detected signal.