PURPOSE: To measure a location of a sedimentation surface of a liquid to be inspected rapidly by a method wherein a transmitter and a receiver for ultrasonic signals are mounted to the bottom of a cylinder body of the sedimentation pipe, and a circuit is fitted which measures the time difference of the transmission and reception of the ultrasonic signals reflecting on the sedimentation surface of the liquid to be inspected and operates.

CONSTITUTION: When a deposit 70 and a supernatant liquid 71 separate in a sedimentation pipe 7, transmission pulses are emitted from a transmission circuit 50, and a transmitter-receiver 11 forwards transmission signals 73 into the deposit 70 while inputting the transmission pulses to a time counter 61. The transmission signals 73 are reflected on a sedimentation surface 72, and reflecting signals 74 are received by means of the transmitter-receiver 11, and inputted to the time counter 61 through a reception circuit 51. The time counter 61 measures the time difference of two signals, and inputs the value to a calculator 62. Temperature signals from a water temperature detector 63 are input to the computing element 62, and a location of the sedimentation surface 72 is calculated on the basis of both signals.