The pH of a substance to be measured is accurately measured without being influenced by the intensity of excitation light (L1), the concentration of fluorescent material and surrounding environments. The pH is measured by generating pulsed excitation-light (L1) including a wavelength that can excite a plurality of kinds of fluorescent-material in living matter that act as coenzyme in oxidation/reduction reaction in vivo, and the intensity of the light not damaging a tissue nor a cell and substantially not changing pH, and by illuminating a predetermined position in the living matter with the light (L1), and by receiving fluorescence (Lf), and by resolving the intensity of the fluorescence (Lf) into time domains the number of which is greater than that of the fluorescent-material, and by detecting the intensities of the fluorescence (Lf) in the respective time domains, and by obtaining approximate-curves having gradients unique to the fluorescent-materials based on the intensities in the respective time domains, and by calculating fluorescence lifetimes (Tf) of at least two of the plurality of kinds of fluorescent-material based on the approximate-curves, and by measuring the pH of the living matter based on the lifetimes (Tf).
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