Abstract: Electrolysis is employed for raising or reducing pH of culture solution. Electrolyte contained in a vessel having at least one ion exchange membrane as a wall is provided so as to confront the culture solution, with the ion exchange membrane lying therebetween. Electrodes are provided both in the electrolyte and the culture solution. With these electrodes the electrolysis is carried out through the ion exchange membrane. Thus formed alkali or acid changes the pH of the culture solution. By controlling the time or supplied current for the electrolysis, required pH can be accurately obtained.