Abstract: Provided is a chemical product production method wherewith filterability and productivity with respect to a target substance can be both achieved. The present invention is provided with a mechanism for removing solid contents in a pipe connecting a fermentation tank and a filter separation module, and adds the solid contents removed by the removal mechanism to a culture liquid.

Abstract: The present invention relates to a method for filtering a microbial culture solution using a membrane module, said method comprising four stages including (a) cross-flow filtration of the microbial culture solution, (b) rinsing with water, (c) washing with a chemical solution and (d) rinsing with water, wherein: the filtrate passing through the membrane module in the cross-flow filtration has a total sugar concentration of 1,000-100,000 mg/1 inclusive and a protein concentration of 50-1,000 mg/l inclusive; the aforesaid stage (c) comprises step (c-1) for washing with a chemical solution containing a hypochlorite and a nonionic surfactant; and the aforesaid stages (a) to (d) are repeated in this order.