PROBLEM TO BE SOLVED: To provide a living body monitoring device capable of receiving weak light emitted from a living body M with a simple configuration and further detecting intensity of the light with high precision to accurately perform various monitoring of the living body M.SOLUTION: A living body monitoring device includes: a light source 10 for radiating primary light L1 whose intensity changes at a prescribed frequency to a living body M and a light receiving unit 20 for receiving secondary light L2 externally emitted from a surface of the living body M by radiating the primary light L1 to detect intensity of the secondary light L2, and the light receiving unit 20 selectively receives the light whose intensity changes at the prescribed frequency.
