In a cooling a galvanized steel sheet (W), change in emissivity of a surface of the galvanized steel sheet (W) is measured in a temperature range lower than the boiling point of zinc but not lower than the ferrite transformation temperature, using an emissivity sensor (4) with an observation wavelength of 1.4 μm or longer, and pressing and quenching in a pressing and quenching apparatus (2) is started after completion of an alloying reaction has been detected based on the change in emissivity. The emissivity sensor (4) preferably has an InGaAs element or a thermopile as a measuring element.