A method of producing a chemical product includes cultivating microorganisms or culture cells in a fermentation tank; transferring a culture liquid from the fermentation tank to a membrane separation tank 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 to be joined to the culture liquid on an upstream side of the membrane separation tank, wherein 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 a culture liquid flow-in side of the membrane separation tank.