The present invention relates to an in-furnace temperature measurement device 1 for a furnace in which gas is generated. The measurement device 1 includes: a support tube 3 which communicates with a measurement hole 2A which leads to an inside of the furnace; a sensor protection tube 4 inserted into the support tube 3 so as to be movable in an axial direction with a leading end, of the sensor protection tube 4, facing a furnace side; a plurality of seal rings 6A and 6B which hermetically seal a gap between the support tube 3 and the sensor protection tube 4 and which are arranged at an interval so as to divide the gap into a plurality of space portions in the axial direction; a temperature sensor 5 housed in the sensor protection tube 4 such that a temperature-sensing portion of the temperature sensor 5 corresponds to a leading-end portion of the sensor protection tube 4; and a drive mechanism 7 which drives the leading-end portion of the sensor protection tube 4 so as to be able to advance and retract in the axial direction with respect to the inside of the furnace through the measurement hole 2A, while maintaining a sealed state by the seal rings 6A and 6B.