Abstract: A substance-in-blood concentration measurement device for measuring concentration of a substance in blood of a living body, including: a target placement unit on which the living body that includes a measurement target portion is placed; a light emission unit that irradiates the measurement target portion with a laser beam; a photodetector that receives reflected light from the measurement target portion and detects intensity of the reflected light; and a condenser lens disposed on an optical path of the reflected light between the measurement target portion and the photodetector. On the optical path from the measurement target portion to the photodetector, in a section from the target placement unit to the photodetector, the reflected light propagates through space, except where transmitted through the condenser lens, and the condenser lens forms an image of the reflected light on the photodetector.