A light emission section spread-spectrum modulates a base band signal at a chip frequency f to thereby generate a primary modulated signal, modulates the primary modulated signal at a frequency 2f to thereby generate a secondary modulated signal, and emits a near-infrared light beam having a specific wavelength into a living organism on the basis of the secondary modulated signal. A light detection section receives a reflected light beam from the interior of the living organism in an effective detection band 2f, converts it to an electrical detection signal, and converts the detection signal to a digital signal at a sampling frequency 4f. The light detection section then demodulates the digital signal at the frequency 2f to thereby generate a primary demodulated signal, and demodulates the primary demodulated signal through spectrum despreading to thereby generate a secondary demodulated signal. Thus, a biological information signal representing biological information is output.