This standard applies to fixed measuring instruments (hereinafter referred to as measuring instruments) for measuring the concentration of L-lactic acid, an optical isomer of lactic acid, present in samples such as blood, fermentation liquid, and food. Enzyme electrode method using a combination of hydrogenated lactate oxidase and an oxygen electrode or hydrogen peroxide electrode, or hexacyanoferrate(II), which is the product of the cytochrome b reaction in the presence of immobilized cytochrome b and hexianoferrate(III) ions. Electrode for electrolytic separation of ions [hereinafter referred to as electrode for hexacyanoferrate(II) ion analysis. ] Specifies the method based on the enzyme electrode method in combination with the method.