To provide a treatment system of a waste liquid containing ready- mixed concrete, which is capable of efficiently recovering gravel and sand from such waste liquid and also the stable functions of which can surely be maintained over a long period.

This system is provided with separators 2, 11, 15 and 19 for separating solids G, S and K from such ready-mixed concrete-containing waste liquid E; storage vessels 8, 12, 16 and 20 for storing liquid fractions E1, E2a, E2b and E3 which are separated by the separators 2, 11, 15 and 19, respectively; a circulation passage 22 formed so that the liquid fraction E3 stored in the storage vessel 20 can be used as cleaning water CW for cleaning the inside of a mixer tank 4a; and a liquid fraction modification treatment device 23 for modifying properties of at least the liquid fraction E3 passing through the circulation passage 22, by using an electric field and a magnetic field, so that the liquid fraction E3 hardly causes any scale deposition.