PURPOSE: To make the effect of bubble hard to receive and to make it possible to perform highly accurate measurement by immersing a dielectric rod in sludge, conducting an electromagnetic microwave through the dielectric rod, and measuring the amount of attenuation of the electromagnetic microwave due to dielectric loss.

CONSTITUTION: Dielectric rod 1 is provided at the central part in a sludge pipe 3 wherein sludge 2 flows along the flowing direction of the sludge 2. One end part of the rod 1 is linked to a transmitting part 5 through a waveguide 4. The other end part of the rod 1 is linked to a receiving part 6. When the sludge concentration is measured, an electromagnetic microwave is conducted through the rod 1 from the transmitting part 5. Then dielectric loss is generated in correspondence with the sludge concentration around the rod 1. Therefore, the electromagnetic microwave is attenuated. The amount of the attenuation of the electromagnetic microwave is received with the receiving part 6. The received signal is transduced into the sludge concentration. The result is displayed on a display circuit. Since the measurement can be performed for the area along the cross section of the rod 1, the accurate measurement can be performed continuously.