A measuring unit for use in a sensor utilizing the phenomenon of attenuation in total internal reflection includes a dielectric block which is transparent to a light beam and has a flat and smooth surface on which a film layer is formed, and a flow passage member held in close contact with the film layer. The flow passage member is provided with a passage comprising a supply passage extending from an inlet of the flow passage member to a measuring portion and a discharge passage extending from the measuring portion to an outlet of the flow passage member.