A novel reactor design incorporating a porous ceramic tubular membrane fitted inside a shake flask was developed for the dense cell culture of microorganisms on an ordinary shaker. The tubular membrane was effective in extracting the culture broth. The filtering performance was found to be enhanced by fitting the filter at the shoulder of the flask. Membrane fouling was prevented for long periods by waves created in the culture broth due to the shaking action, which constantly washed the membrane surface. Using this shake flask system, the concentration of Saccharomyces cerevisiae cell mass reached 235 g/l at 220 h with the feeding of fresh medium and extraction of the supernatant. The long-lasting and high permeability performance of the ceramic membrane enabled increases in the concentration and productivity of viable cells to be readily achieved by continuously refreshing the culture supernatant.