We have developed a high efficiency L-lactate fermentation process employing the homofermentative lactic acid bacterium, Lactococcus lactis IO-1 with a pH-dependent substrate feed system in order to consume the substrate completely and maintain high activity of cell growth, production, and substrate consumption. Furthermore a pH-dependent substrate feed system was combined with turbidity control consisting of a laser probe and micro filter module for cell-recycling. With this system, we achieve a high efficiency L-lactate continuous culture. We are able to construct high efficiency bioconversion systems from xylose to P (3 HE) via L-lactate and other organic acids because Lactococcus lactis IO-1 can utilize not only glucose but also xylose as a carbon source. Thus, we have developed a new substrate feed system which is able to automatically control substrate concentration in a broth. A new batch fermentative system for production of biodegradable plastic material, polyhydroxybutyrate [P (3 HE)] from culture broth of Lactococcus lactis IO-1 is vealized.