L-lactic acid was produced by direct fermentation of potato using Lactobacillus amylophilus which has both the ability to saccharify starch and produce L-lactic acid. Potato was found to be a more suitable fermentation substrate compared to purified potato starch. It is estimated that protein and oligosacchalide which are contained in potato contribute as fermentation promoters. The utilization of squid entrails or scallop entrails as culture nutrients instead of polypeptone and yeast extract is extiemely successful. In particular, fermentation in squid entrails is strongly promoted, and is considered to come from enhancement of saccharification ability of Lactobacillus amylophilus.