In order to enable the measurement of thermal property information about a subject, this thermal diffusion coefficient measuring device, which is used by contacting the surface of a living body, is provided with: a biological information sensor comprising a temperature sensor and a heat flux sensor; and a heating/cooling control means. The temperature sensor is provided at a position contacting the surface of the living body, and operates so as to detect skin temperature. The heat flux sensor is provided at a position contacting the surface of the living body, while being adjacent to the temperature sensor, and operates so as to detect heat flux on the surface of the living body. The heating/cooling control means enables the measurement of the temperature diffusion coefficient of a thermal resistance component that is present between the biological information sensor and a deep inner portion of the living body.