A concentration measurement method by an absorptiometric method which measures a concentration of a first solute component dissolved in a solution, in which the first solute component interacts with a solvent, includes: referring to an absorption coefficient (&mu；aw(&lambda；)) of the solvent； measuring an absorption coefficient (&mu；a(&lambda；)) of a first sample material, after an interaction between the solvent and the first solute component has occurred； and obtaining an unknown volume fraction (vg) of the first solute component and a volume fraction (vw) of the solvent, based on the absorption coefficient (&mu；aw(&lambda；)) of the solvent, a apparent absorption coefficient (&mu；&prime；ag(&lambda；)) of the first solute component, and the absorption coefficient (&mu；a(&lambda；)) of the first sample material.
