A temperature measuring device for a rotating roll comprises a housing with guide rollers around which a belt is looped and held under tension by an idler roll. One boundary of the housing is arched concavely in accordance with typical roll diameters to enable the blet to run freely in this region outside the housing and press against a roll surface in a looping region to drive the belt as the roll rotates. The inner side of the belt facing away from the roll may be blackened and the energy radiated therefrom may be measured by a radiation pyrometer to determine the temperature of the roll.