PURPOSE: To realize a countinuous detection of the sludge height by detecting the sludge height with reception of the ultrasonic wave reflected from the upper surface of the sludge via the reflective ultrasonic transmitter/receiver.

CONSTITUTION: Ultrasonic transmitter/receiver 20 is hanged down into tank 15 from the upper part of tank 15 at the measuring time to be positioned on the same level as the supernatant. Then the ultrasonic wave is transmitted to the sludge, and the reflected wave is received. The time shift is measured between the transmission and reception at transmission and indication part 21 to indicate distance X1 from the water level to the sludge surface. At the same time, water level X2 of supernatant in tank 15 is measured at water level indication part 23. Then position XS of the sludge surface is calculated based on water level indication value X2 obtained from part 23 and distance indication value X1 obtained from part 21.