To provide a method for treating waste water generated when gravel is washed, by which efficient settling speed can be achieved regardless of the concentration of a suspended material in the waste water generated when the gravel is washed.

After a polycondensation type cationic material is added to inorganic particle-suspended water such as the waste water generated when the gravel is washed and then mixed, an anionic water-soluble macromolecule consisting of a dispersed liquid of fine particles of a macromolecule, that is manufactured by a dispersion polymerization method in a salt aqueous solution in the coexistence of a salt aqueous solution-soluble macromolecular dispersant and has ≤100 μm particle size, is added to the cationic material-mixed waste water, mixed, coagulated and then subjected to solid-liquid separation.