An extractive fermentation system was developed to prevent end-product inhibition of Clostridium acetobutylicum I AM 19012, which mainly produces butanol and acetone. Butanol exhibited greater toxicity to the microorganism than acetone, and its growth was completely inhibited above 10kg/m³ of butanol. As an extracting solvent suitable for acetone-butanol fermentation, oleyl alcohol (cis-9-octadecen-l-ol) and C-20 guerbet alcohol (branched-chain alcohol of carbon number, 20) were selected from among 29 organic compounds, based on their nontoxicity to the microorganism. These two solvents had high partition coefficients for butanol, and could be reused without deterioration. In fermentation with the solvent (solvent phase: aqueous phase = 2:5 (v/v)), the viability of the microorganism was resumed by the liquid-liquid extraction of butanol from the broth, and the amount of butanol produced was 2.6 times that in fermentation without extraction.