PURPOSE: To achieve a higher detection accuracy of the width-wise edge of heated metal ingot by making the scanning cycle of a photo detector provided at the width-wise edge of the metal ingot alterable based on the signal of a photo detector provided separately at a width-wise edge.

CONSTITUTION: A photo detector 13 is arranged between a photodetector 9 on the work side and a photodetector 10 on the drive side to permit the detection of the quantity of light at the specified position of a rolling material (not illustrated) and signals of the photodetectors 9 and 10 are compared with the threshold level with arithmetic circuits 17 and 18 through amplifiers 15 and 16 to feed an ON/OFF signal to buffer amplifiers 19 and 20. In addition, the signal of the photodetector 13 is fed to a conversion circuit 21 to generate a pulse proportional to the quantity of light of the rolling material, which is counted with a timing pulse generation circuit 22 to alter the scanning times of the photodetectors 9 and 10. Thus, a detection can be done at the optimum timing according to the temperature of the rolling material.