The object of the present invention is to provide a method for measuring concentration of a biological substance contained in a living body in which deterioration of the accuracy due to the reflected light and the interruption component is suppressed. Linear-polarized light is emitted to a particle chip implanted in the skin with modulating its modulating direction continuously. A surface enhanced Raman scattering light of the biological substance generated on the particle chip. A concentration of the biological substance is calculated based on the received signal. The receiving signal satisfy the following equation:R(t)=Am ·sin(ωt)+D, where R(t): received signal, Am: amplitude, t: time, D: a constant number, and ω: angular speed.
