PURPOSE: To enable adequate replacement of a thermocouple temperature sensor by a method wherein a resistance value being the sum of a resistance of a thermocouple and a wiring resistance is computed from an output signal of a sensor and a difference between the initial value thereof and the present resistance value is determined.

CONSTITUTION: A temperature and others in a furnace wherein a thermocouple temperature sensor 1 is used are computed in a temperature computing element 7 from a digital output signal of the sensor 1. A resistance value computing element 8 computes a resistance value being the sum of a resistance R1 of a thermocouple and a wiring resistance R2, from this digital output signal and the temperature computed in the computing element 7. A resistance value hysteresis keeping element 9 stores and keeps the initial value of the resistance value computed in the computing element 8 and a resistance value sampled intermittently, and a threshold resistance value storage element 10 stores a threshold resistance value showing a change in the resistance at the time of deterioration of each kind of thermocouple. A lifetime determining element 11 computes a difference between the present resistance value obtained in the computing element 8 and the initial value in the keeping element 9, compares the value of this difference with the threshold resistance value, and outputs a signal to a notice function element 14 when the value of the difference exceeds the threshold resistance value.