An apparatus and method for measuring a temperature of a high temperature liquid contained in a furnace. An optical fiber covered with a metallic tube is inserted through a passageway inside a nozzle arranged on a furnace wall of the furnace. The nozzle communicates with an interior of the furnace containing the liquid, and gas is supplied into the passageway inside the nozzle to prevent the nozzle from clogging. The metal-covered optical fiber is fed through the passageway inside the nozzle into the liquid such that spectral light radiated from the liquid enters a tip of the metal-covered optical fiber and is propagated therealong. The temperature of the liquid is determined by a radiation thermometer, coupled to the metal-covered optical fiber, based on the spectral light propagated along the metal-covered optical fiber.