An L-ribose isomerase which isomerizes aldoses such as L-ribose, D-lyxose, D-talose, D-mannose, L-allose and L-gulose into their corresponding ketoses such as L-ribulose, D-xylulose, D-tagatose, D-fructose, L-psicose and L-sorbose. The enzymatic reaction is a reversible equilibrium reaction. The L-ribose isomerase can be obtained from microorganisms of the genus Acinetobacter.