PURPOSE: To detect the boundary surface with high accuracy by an apparatus wherein an ultrasonic wave is transmitted to a boundary float floating at the boundary between the solution and the solvent, and a time period until receipt of the reflected wave is measured.

CONSTITUTION: A boundary float 108 is formed of synthetic resin material to be floated at a position of the boundary 105. The boundary float 108 is shaped into a star-like or other form in plan view, so that a solution 103 and a solvent 104 separated from eachother are not prevented from floating. A reflection plate 109 formed of corrosion resistanct metal material is attached at the upper central surface of the float 108 to effectively reflect a ultrasonic wave from a ultrasonic transducer 111. An ultrasonic wave radiated downward through a solvent float 110 is reflected by the boundary float 108 and the bottom surface of an extracting tank 101. Thus reflected wave is received by the ultrasonic transducer 111, and positions of the boundary float 108 and the solvent float 110, i.e., levels of the boundary 105 and the liquid surface of the solvent 104, is calculated by a signal processing circuit 114 based on a time period until receipt of the reflected wave.