To prevent slurried flocculate from being included into a recovered polishing agent.

This separation device comprises a first filter 1 with an ultrafiltration membrane 1m and a second filter 2 with a precision filtration memberane 2m which are connected to each other in that order. First, fine particle contained in a polishing liquid L is discharged to the first filtrate Fp side and a first concentrated Po is supplied to the second filter 2 by separating a used polishing liquid L into the first Fp and the first concentrated liquid Po with the filtrate ultrafiltration membrane 1m. Next, a coarse particle contained in the first concentrate Fc is discharged to a second concentrate Sc side and a second filtrate Sp is supplied to a recovery side by separating the first concentrate Fc into the second concentrate Sc and the second filtrate Sp with a precision filtration element 2m.