A pH measuring method comprising: illuminating a medium solution with a plurality of light beams, the medium solution including: a first material which, in a wavelength region of 300 nm to 800 nm, has a first absorption peak; and a second material which, in the wavelength region of 300 nm to 800 nm, has a second absorption peak or a second convergence point where an absorbance is converged irrespective of a pH, and a third absorption peak or a third convergence point where an absorbance is converged irrespective of a pH, the plurality of light beams including: a first light beam, a wavelength of which corresponds to the first absorption peak; a second light beam, a wavelength of which corresponds to the second absorption peak or the second convergence point; a third light beam, a wavelength of which corresponds to the third absorption peak or the third convergence point; and a fourth light beam, a wavelength of the fourth light beam at which an absorbance of at least one of the first material and the second material is converged irrespective of a pH; receiving transmitted or reflected light beams of the first to fourth light beams; measuring absorbances at the wavelengths of the received transmitted or reflected light beams of the first to fourth light beams respectively; and calculating a pH of the medium solution based on the measured absorbances. wherein the pH of the medium solution is calculated with an expression including a sum of the absorbance of the first material and the absorbance of the second material. illuminating a medium solution with a plurality of light beams, the medium solution including: a first material which, in a wavelength region of 300 nm to 800 nm, has a first absorption peak; and a second material which, in the wavelength region of 300 nm to 800 nm, has a second absorption peak or a second convergence point where an absorbance is converged irrespective of a pH, and a third absorption peak or a third convergence point where an absorbance is converged irrespective of a pH, the plurality of light beams including: a first light beam, a wavelength of which corresponds to the first absorption peak; a second light beam, a wavelength of which corresponds to the second absorption peak or the second convergence point; a third light beam, a wavelength of which corresponds to the third absorption peak or the third convergence point; and a fourth light beam, a wavelength of the fourth light beam at which an absorbance of at least one of the first material and the second material is converged irrespective of a pH; a first material which, in a wavelength region of 300 nm to 800 nm, has a first absorption peak; and a second material which, in the wavelength region of 300 nm to 800 nm, has a second absorption peak or a second convergence point where an absorbance is converged irrespective of a pH, and a third absorption peak or a third convergence point where an absorbance is converged irrespective of a pH, a first light beam, a wavelength of which corresponds to the first absorption peak; a second light beam, a wavelength of which corresponds to the second absorption peak or the second convergence point; a third light beam, a wavelength of which corresponds to the third absorption peak or the third convergence point; and a fourth light beam, a wavelength of the fourth light beam at which an absorbance of at least one of the first material and the second material is converged irrespective of a pH; receiving transmitted or reflected light beams of the first to fourth light beams; measuring absorbances at the wavelengths of the received transmitted or reflected light beams of the first to fourth light beams respectively; and calculating a pH of the medium solution based on the measured absorbances. wherein the pH of the medium solution is calculated with an expression including a sum of the absorbance of the first material and the absorbance of the second material.