A field-flow fractionation device comprising: a separation channel provided with an inlet port and an outlet port at both ends thereof, the separation channel forming a space through which a carrier fluid is to flow; a carrier fluid supplier (14, 15) which is configured to supply the carrier fluid to the separation channel via the inlet port; a separation membrane which is configured to block particles to be separated from passing therethrough while passing the carrier fluid therethrough, the separation membrane forming a wall surface defining the separation channel; a waste liquid chamber which forms a space in which the carrier fluid having passed in the separation channel through the separation membrane flows, the waste liquid chamber having a discharge port for discharging the carrier fluid having passed through the separation membrane to the outside; and a cross-flow flow rate adjuster connected to the discharge port, the cross-flow flow rate adjuster being configured to adjust a flow rate of a cross flow formed by the carrier fluid passing through the separation membrane from the separation channel by adjusting a flow rate of the carrier fluid discharged from the waste liquid chamber via the discharge port to a preset flow rate, wherein the field-flow fractionation device is characterized by further comprising a carrier fluid adder which is configured to add another flow of the carrier fluid to a flow of the carrier fluid having passed through the separation membrane at a carrier fluid adding position set on an upstream of the cross-flow flow rate adjuster so that the flow rate of the carrier fluid flowing into the cross-flow flow rate adjuster becomes larger than the flow rate of the carrier fluid having passed through the separation membrane. a separation channel provided with an inlet port and an outlet port at both ends thereof, the separation channel forming a space through which a carrier fluid is to flow; a carrier fluid supplier (14, 15) which is configured to supply the carrier fluid to the separation channel via the inlet port; a separation membrane which is configured to block particles to be separated from passing therethrough while passing the carrier fluid therethrough, the separation membrane forming a wall surface defining the separation channel; a waste liquid chamber which forms a space in which the carrier fluid having passed in the separation channel through the separation membrane flows, the waste liquid chamber having a discharge port for discharging the carrier fluid having passed through the separation membrane to the outside; and a cross-flow flow rate adjuster connected to the discharge port, the cross-flow flow rate adjuster being configured to adjust a flow rate of a cross flow formed by the carrier fluid passing through the separation membrane from the separation channel by adjusting a flow rate of the carrier fluid discharged from the waste liquid chamber via the discharge port to a preset flow rate, wherein the field-flow fractionation device is characterized by further comprising a carrier fluid adder which is configured to add another flow of the carrier fluid to a flow of the carrier fluid having passed through the separation membrane at a carrier fluid adding position set on an upstream of the cross-flow flow rate adjuster so that the flow rate of the carrier fluid flowing into the cross-flow flow rate adjuster becomes larger than the flow rate of the carrier fluid having passed through the separation membrane.