To accurately detect the position of an interface between a sledge and supernatant water.

An interface level meter includes: a sensor 2 for emitting an ultrasonic wave or light and receiving the ultrasonic wave or the light reflected under water containing a suspended matter deposition layer; and a calculation part 6 for calculating the position of an interface between the suspended matter deposition layer and supernatant water. The calculation part 6 compares the intensity of the reception signal after the elapse of the predetermined time set by the position of a foreign matter under water from the emission of the ultrasonic wave or light and the predetermined threshold value, and calculates the position of the interface based on the comparison result.