Glass is heated from above and below while the glass resides on rolls (3) in a tempering furnace (1). The upper surface of the glass (4) is heated by hot air jets formed by sucking air from inside the furnace (1) and pressurizing the hot air and recycling it back to the upper surface of the glass. Air which has been taken from outside the furnace (1) and pressurized by a compressor (17) and heated is blown to the lower surface of the glass.