Abstract: A measuring sensor for use in determination of the heat flux through a solid medium according to the method in which two thermoelements are arranged at a defined distance and a temperature gradient of the medium is measured from which the heat flux is determined, composed of a body shaped to be fitted into an opening provided in the medium to be axially penetrated by the heat flux through the medium while the lateral surface of the body is thermally insulated from the medium, and constructed such that the integrated coefficient of thermal conductivity of the body is substantially equal to the coefficient of thermal conductivity of a volume of the medium having dimensions corresponding to those of the opening, the body including components of two different thermoelectric materials defining at least two spaced thermoelectrically effective contact points, and signal leads associated with the contact points and spaced apart in the direction of the heat flux; and wherein the exterior lateral surface of said body i