To improve the responsiveness when the temperature of the steel sheet is changed without greatly increasing the heating cost of the steel plate.

In heat treating a steel sheet in a continuous annealing facility of the steel sheet provided with a radiation tube heating furnace, a direct-fired heating furnace at the inlet side of the radiation tube heating furnace, and a preheating furnace 11 using the combustion exhaust gas of the direct-fired heating furnace on the inlet side of the direct-fired heating furnace, an induction heating device 17 is installed at least between the direct-fired heating furnace and the preheating furnace, or between the direct-fired heating furnaces where a plurality of direct-fired heating furnaces 15, 19 are provided, the steel plate is mainly heated using the direct-fired heating furnace and the radiation tube heating furnace 21, and heat-treated by compensating the response delay or insufficient heating capacity by the induction heating in the case of the response delay of the temperature control in changing the heating load of the direct-fired heating furnace and the radiation tube heating furnace or in the case of insufficient heating capacity thereof. The problem of oxidation of the steel plate surface is solved, an induction heating equipment of large capacity is dispensed with, and the heating cost by induction heating is less increased.