PURPOSE: To provide an in-furnace temperature measuring device by which even a temperature inside a high-temperature furnace heated to a high temperature of 1000 deg.C or higher can be measured precisely. CONSTITUTION: A heating heater and an out-furnace temperature measuring device are installed at the outside of a furnace body, and an in-furnace temperature measuring device 20 is installed at the inside of the furnace body. Before a manufacturing operation, the furnance body is heated by the heating heater, temperatures at the inside and the outside of the furnace body are measured, and calibration data is created. In an actual manufacturing operation, a heating operation is controlled by the measured temperature of only the out-furnace temperature measuring device. At the in-furnace temperature measuring device 20, a protective tube 21 composed of a silicon carbide or alumina is installed so as to protrude at the inside of the furnace body. Thermocouple strands 22a, 22b are constituted in such a way that they are covered with insulating tubes 23a, 23b so as to be inserted into, and passed through, the protective tube 21.