In a method for producing a cell concentrate using a cell suspension treatment system including a storage container of a cell suspension, which has a solution inlet port, a circulation outlet port, and a circulation inlet port, a cell suspension treatment device for concentrating the cell suspension by separating liquid from the cell suspension by filtration, the device including a container having a cell suspension introduction port, a cell suspension lead-out port, and a filtrate outlet, which is filled with a hollow fiber separation membrane, a circulation circuit for concentrating the cell suspension while circulating the cell suspension between the storage container and the cell suspension treatment device, a collection container of a cell concentrate obtained by concentration, a collection path for feeding the cell concentrate to the collection container, an injection path for injecting a solution into the solution inlet port of the storage container, and a detecting unit.