PURPOSE: To detect only a heated steel plate, by transducing radiated heat energy into a voltage with a detecting element, obtaining the voltage change per unit time, and comparing the result with a voltage change corresponding to a preset temperature change per unit time.

CONSTITUTION: Infrared rays emitted from a heated steel plate 17, which moves on rollers 16 on a conveying line, are condensed through a head 1 and guided through fluoride fiber 2. The light emitted from the fiber 2 is turned on and off by a rotary chopper 9 and inputted into a PbS detector 3. The incident infrared-ray energy is transduced into a voltage signal through the detector 3. The signal is amplified as AC in an amplifier 4. The AC is converted into the DC in an AC/DC converter circuit 5. The DC is sent to a differential circuit 6. The circuit 6 is constituted with a capacitor, resistor and the like. When the voltage signal is inputted into the input side, the voltage change per unit time is outputted on the output side. Then the voltage change is compared with a voltage change per unit time corresponding to the temperature change per unit time, which is set in a reference voltage source 8 in the next comparator 7. When the measured voltage change is larger than the reference value, a relay output contact point (c) is turned ON.