Product production method

A product production method, comprising:

• a step of culturing a cell which produces a product and is contained in a cell suspension accommodated in a culture vessel;
• a separation treatment step of extracting the cell suspension from the culture vessel, and separating the cell suspension, using a separation membrane, into a return liquid having a cell concentration higher than that of the cell suspension and a permeated liquid having a cell concentration lower than that of the cell suspension and containing the product by a tangential filtration method;
• a step of returning the return liquid to the culture vessel;
• a step of supplying a fresh medium into the culture vessel;
• and
• a step of collecting the product,
• wherein in a case where a live cell concentration, which is expressed in a unit of cells/mL, in the cell suspension is denoted by Nc, a pore diameter, which is expressed in a unit of m, of the separation membrane is denoted by Dp, a filtration area, which is expressed in a unit of m2, of the separation membrane is denoted by S, and a volume, which is expressed in a unit of cm3, of a primary side flow path of the separation membrane is denoted by Vf, a number density Nd, which is expressed in a unit of particles/mL, of fine particles having a particle size of 8 Dp to 30 Dp other than live cells in the cell suspension satisfies Expression (1),
• Nc≤Nd≤S/(32×π×Vf×Dp2) Expression (1).