Provided is a simple method for producing a cell concentrate (other than human blood) in a short time with neither loss of cells nor an excessive burden on cells. Included is a method for producing a cell concentrate using an inside-out filtration system for processing a cell suspension, the system including: a cell suspension inlet port; a filtrate outlet port; a cell suspension outlet port; and hollow fiber separation membranes provided between the cell suspension inlet port and the cell suspension outlet port, wherein the membranes have a total cross-sectional area of 0.5-1.5 cm2, the membranes have an inside membrane area of 0.2 m2 or less, the suspension is flowed inside the membranes at a linear velocity of 500-1200 cm/min, and the quotient of the division of the initial filtrate flow rate from the filtrate outlet port by the flow rate into the cell suspension inlet port is 0.4-0.7.