The radiation thermometer includes: a sensor producing the first signal of applied power source and the second signal of the IR radiation of target object, a chopper between the target object and thesensor, a electric excitor applying electric energy to the sensor, and a signal processing unit, which processes the said signals to find out the IR radiance received by the sensor and the temperature of target object. The sensor includes two sensor elements reversely connected, serially or parallelly, to eliminate their noise.