A gas is supplied to the surface of the glass sheet transferred between a heating furnace and a cooling apparatus for tempering the glass sheet. The gas is supplied from the upstream side with respect to the direction of transfer of the glass sheet by an air curtain device. This gas runs along the glass sheet surface (upper surface) downstream with respect to the direction of transfer to obstruct the flow of cooling gas from the cooling apparatus in the upstream direction. The gas prevents cooling air from the cooling apparatus from invading the heating furnace so that the difference of the curvature of the glass sheet on the front and rear part can be eliminated.