Field flow fractionation device includes a channel switching unit for switching the connection of a second carrier fluid supply unit to any one of the second inlet port of an upper separation cell, the first inlet port of a lower separation cell, or the second inlet port of a lower separation cell. Furthermore, the second carrier fluid supply unit is connected to the second inlet port of an upper separation cell during the process of focusing to generate flow of carrier fluid counter to the flow of carrier fluid from the first inlet port within the upper separation cell, whereas the second carrier fluid supply unit is connected to the first inlet port or the second inlet port of a lower separation cell after conclusion of focusing in the upper separation cell.