To provide a washing method of B/F separation and a washing device of B/F separation capable of washing efficiently with a reduced washing liquid amount as a whole, and reducing an influence of a washing liquid residue.

When B/F separation is carried out, for the first time, discharge and successive draining are performed so as to acquire washing liquid having a prescribed concentration; for the second time and thereafter, discharge and successive draining are performed so as to acquire thinner washing liquid than the case for the preceding time; and at the final round, discharge and successive draining are performed so as to acquire thinner washing liquid than the concentration at the first time.