A circuit diagram is proposed on the basis of an analysis of metabolic pathways of lactic acid bacteria, namely, a phosphoketolase pathway and a pentose phosphate/glycolic pathway. An augmented matrix was derived from carbon balances and stoichiometries from the circuit diagram, and solved by Gaussian elimination. Results indicated that 70% of the amount of xylose was metabolized through the pentose phosphate/glycolic pathway and that the yield of lactic acid from xylose ranged from 0.34 to 0.52 (mol/mol), whereas that of pyruvic acid was more than 1.4 (mol/mol).