A device for measuring the concentration of a biological constituent based on infrared radiation emitted by a subject's eardrum with the influence of the eardrum's thickness taken into account is provided.The biological constituent concentration measuring device includes: a detecting section for detecting infrared radiation emitted by an eardrum; an acquisition section for acquiring thickness information about the thickness of the eardrum; and a computing section for figuring out the concentration of the biological constituent based on the infrared radiation detected and the thickness information acquired. The infrared radiation emitted by the eardrum is subject to the influence of the subject's eardrum thickness. Therefore, by calculating the biological constituent concentration based on not only the infrared radiation detected but also the eardrum thickness information, the biological constituent concentration can be measured highly accurately.