A method of producing a chemical includes culturing cells in a culture solution in a fermentor to ferment a feedstock to produce a chemical; supplying the culture solution containing the chemical produced in the culturing to a plurality of separation membrane units arranged in parallel; filtering the culture solution supplied in the supplying to separate a permeate containing the chemical; refluxing a retentate that is not filtered in the filtering to the fermentor; and supplying a gas containing oxygen to the plurality of separation membrane units while a supply amount is changed to at least two different values to perform scrubbing, wherein the supply amount and supply time of the gas containing oxygen supplied in the culturing and the supplying the gas are set so that a kLa value is within a predetermined range from an optimal kLa value for the cells cultured in the culturing.