The present invention provides a method that allows the preparation of a cell concentrate in a short time without loss of cells and great damage on cells by simple operations. The present invention provides 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 a hollow fiber separation membrane interposed between the cell suspension inlet port and the cell suspension outlet port, wherein the hollow fiber separation membrane is provided with inner pores with an average pore size of 0.1 μm to 10 μm, and the quotient of the division of an initial filtrate flow rate by a linear velocity of the cell suspension flowing through the hollow fibers is 2.5 or less.
