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 (μaw(λ)) of the solvent measuring an absorption coefficient (μa(λ)) 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 (μaw(λ)) of the solvent, a apparent absorption coefficient (μ′ag(λ)) of the first solute component, and the absorption coefficient (μa(λ)) of the first sample material.
