A method for producing a cell concentrate: using an inside-out filtration system for processing a cell suspension, the system comprising: a cell suspension inlet port; a filtrate outlet port; a cell suspension outlet port; and a set of hollow fiber separation membranes provided between the cell suspension inlet port and the cell suspension outlet port, wherein the hollow fiber separation membranes have a total cross-sectional area, which is the total of the cross-sectional areas of the lumens of the hollow fiber separation membranes, of 0.5 cm 2 to 1.5 cm 2, the hollow fiber separation membranes have an inside membrane area, which is the total of the inside areas of the hollow fiber separation membranes, of 0.2 m 2 or less, causing a cell suspension to flow inside the hollow fiber separation membranes at a linear velocity of 500 cm/min to 1000 cm/min, which is the quotient of the division of the amount of fluid flowing into the lumens of the hollow fiber separation membranes per unit time by the total cross-sectional area of the hollow fiber separation membranes, and a quotient of the division of an initial filtrate flow rate, which is the filtrate flow rate per unit time used to filter an initial 500 mL filtrate at the beginning of the treatment, from the filtrate outlet port by a flow rate into the cell suspension inlet port is 0.4 to 0.7. using an inside-out filtration system for processing a cell suspension, the system comprising: a cell suspension inlet port; a filtrate outlet port; a cell suspension outlet port; and a set of hollow fiber separation membranes provided between the cell suspension inlet port and the cell suspension outlet port, wherein the hollow fiber separation membranes have a total cross-sectional area, which is the total of the cross-sectional areas of the lumens of the hollow fiber separation membranes, of 0.5 cm 2 to 1.5 cm 2, the hollow fiber separation membranes have an inside membrane area, which is the total of the inside areas of the hollow fiber separation membranes, of 0.2 m 2 or less, a cell suspension inlet port; a filtrate outlet port; a cell suspension outlet port; and a set of hollow fiber separation membranes provided between the cell suspension inlet port and the cell suspension outlet port, wherein the hollow fiber separation membranes have a total cross-sectional area, which is the total of the cross-sectional areas of the lumens of the hollow fiber separation membranes, of 0.5 cm 2 to 1.5 cm 2, the hollow fiber separation membranes have an inside membrane area, which is the total of the inside areas of the hollow fiber separation membranes, of 0.2 m 2 or less, causing a cell suspension to flow inside the hollow fiber separation membranes at a linear velocity of 500 cm/min to 1000 cm/min, which is the quotient of the division of the amount of fluid flowing into the lumens of the hollow fiber separation membranes per unit time by the total cross-sectional area of the hollow fiber separation membranes, and a quotient of the division of an initial filtrate flow rate, which is the filtrate flow rate per unit time used to filter an initial 500 mL filtrate at the beginning of the treatment, from the filtrate outlet port by a flow rate into the cell suspension inlet port is 0.4 to 0.7.