A concentration measurement method for measuring a concentration of impurities includes a step of irradiating a DUT 10 serving as a measurement target object with measurement light and stimulus light subjected to intensity modulation using a modulation signal including a default frequency, a step of outputting a detection signal by detecting an intensity of reflected light from the DUT 10 or transmitted light through the DUT 10, and a step of detecting a phase delay of the detection signal with respect to the modulation signal, obtaining a frequency at which the phase delay has a predetermined value, and estimating a concentration of impurities in the measurement target object on the basis of the frequency.