PURPOSE: To obtain a method of simply measuring sulfuric acid concentration of a lead-acid battery or the like by arranging a humidity sensor in a gaseous phase in the state of equilibrium with a sulfuric acid liquid, and measuring humidity in the gaseous phase so as to convert it into the sulfuric acid concentration.

CONSTITUTION: A gaseous phase is secured in the upper part inside a tightly closed-up container containing a sulfuric acid liquid and a humidity sensor is arranged in this gaseous phase. Then, the humidity measured by the humidity sensor is taken outside the container as an electric signal for being displayed as sulfuric acid concentration by a conversion circuit. In this way, the humidity in the gaseous phase in the equilibrium state with a sulfuric acid liquid is measured as an electric signal so as to be directly displayed as sulfuric acid concentration in order to obtain a method to simply measure sulfuric acid concentration inside a lead-acid battery or the like.