1. A measurement method of measuring a spectroscopic characteristic inside of a scattering medium, said measurement method comprising: a first step of measuring the spectroscopic characteristic of the scattering medium by using diffuse optical tomography by irradiating light into the scattering medium; a second step of measuring the spectroscopic characteristic of the scattering medium by using acousto-optical tomography or photo acoustic tomography by irradiating light into the scattering medium; and a third step of making an assumption of a distribution of the spectroscopic characteristic inside of the scattering medium and of changing the assumption such that a difference between a predicted value of the spectroscopic characteristic derived from the assumption and a measured value obtained in the first step can fall upon a permissible range, wherein the third step uses data obtained in the second step for at least one of the parameter: an initial value, a constraint condition, or a boundary condition.