To improve a treatment method of flocs obtained by adding a coagulant to residual liquid obtained after removing gravels, clay, sand and the like from mud and muddy water generated in underground excavation works and underground continuous wall construction works by the mud and muddy water treatment apparatus to coagulate sludge.

In the treatment method of the flocs obtained by adding the coagulant to the residual liquid after removing gravels, clay, sand and the like by the mud and muddy water treatment apparatus to coagulate and precipitate sludge, a vibrating screen which is used for removing the gravel, clay, sand and the like in the preceding process is also used to dewater flocs.