<p>A galvanized steel sheet (W) is cooled, during which a surface of the galvanized steel sheet (W) is examined for change in emissivity in the range of temperatures which are lower than the boiling point of the zinc but not lower than the ferrite transformation temperature by means of an emissivity sensor (4) for wavelengths of 1.4 µm and longer. Pressing and quenching in a pressing/quenching device (2) are started after completion of the alloying reaction is detected on the basis of a change in emissivity. The emissivity sensor (4) preferably is a device in which the sensing element is an InGaAs element or a thermopile.</p>
