PURPOSE: To make it possible to control an aggregation state of fats and oils in a waste liquid by providing a sensor for sensing electric conductivity of the waste liquid in a tank, where aggregation of fats and oils in the waste liquid is effected, to directly sense the aggregation state of the fats and oils in the waste liquid.

CONSTITUTION: A waste liquid treatment device has an aggregation tank 4, where activated clay is fed into a waste liquid 1 and pH control of the liquid 1 is conducted, and a vacuum filter 7 for removing fats and oils aggregated in the tank 4 using a filter drum 12. An electric conductivity sensor 6 for sensing electric conductivity of waste liquid is provided in the tank 4. In this manner, an aggregation state of fats and oils caused in a waste liquid treatment process can be directly detected by the sensor 6, and hence an aggregation agent such as activated clay can be added according to this aggregation state, so that the amount of the agent to be added can be optimized and further troubles such as flowing-out of fats and oils can be obviated.