To provide a concentration measurement method with which the concentrations of predetermined chemical components can be measured non-destructively, accurately, and rapidly by a simple means, up to the concentrations in trace amount ranges, as well as a concentration measurement method with which the concentrations of chemical components in a measurement target can be accurately and rapidly measured in real time up to the concentrations in nano-order trace amount ranges, and which is endowed with a versatility that can be realized in a variety of embodiments and modes. In the present invention, a measurement target is irradiated, in a time sharing manner, with light of a first wavelength and light of a second wavelength that have different optical absorption rates with respect to the measurement target. The light of each wavelength, arriving optically via the measurement target as a result of irradiation with the light of each wavelength, is received at a shared light-receiving sensor. A differential signal is formed, the differential signal being of a signal pertaining to the light of the first wavelength and a signal pertaining to the light of the second wavelength, the signals outputted from the light-receiving sensor upon receipt of the light. The concentration of a chemical component in the measurement target is derived on the basis of the differential signal.