To control flow velocity of a culture liquid inside a membrane separation tank without giving influences to culture conditions in the fermentation tank, and also suppress precipitation of microorganisms or culture cells so that the production efficiency of the chemical product can be improved, in a method for producing a chemical product including the steps of: cultivating microorganisms or culture cells in a fermentation tank; transferring a culture liquid from the fermentation tank to a membrane separation tank so as to filter the culture liquid through a separation membrane; and collecting a fermentation product from a filtration liquid as the chemical product while refluxing an unfiltered culture liquid that has not been filtered so as to be joined to the culture liquid on an upstream side of the membrane separation tank, one portion of the culture liquid to be transferred from the fermentation tank is allowed to bypass the membrane separation tank depending on a pressure at the culture liquid flow-in sid
