Method of producing chemical by continuous fermentation and continuous fermentation apparatus

A method of producing a chemical comprising:

• a chemical production step of culturing cells in a culture solution in a fermentor to ferment a feedstock to produce a chemical;
• a culture solution supply step of supplying the culture solution containing the chemical produced in the chemical production step to a plurality of separation membrane units arranged in parallel;
• a filtration step of filtering the culture solution supplied in the culture solution supply step to separate a permeate containing the chemical;
• a reflux step of refluxing a retentate not filtered in the filtration step to the fermentor;
• and
• a washing step of washing the plurality of separation membrane units by scrubbing with a gas containing oxygen and having at least two different flow rates for the plurality of separation membrane units while supplying the culture solution to the plurality of separation membrane units, wherein
• the supply amount and supply time of the gas containing oxygen supplied in the chemical production step and the washing step are set so that a kLa value is within a predetermined range from an optimal kLa value for the cells cultured in the chemical production step.