To provide an improved type oven, a tunnel oven especially for mass production of PDP.

This heat treatment device is constituted so as to include a means for recirculating heated fluid and to change periodically the flow direction of the heated fluid during heat treatment, and includes a heating chamber equipped with at least first and second regions and a pump equipped with a suction chamber and a discharge duct. The duct and the suction chamber are communicated by the fluid with the heating chamber. The device also includes a fluid heater arranged on the upstream or the downstream of the pump, and the first diverter means for guiding the heated fluid separated from the pump through the discharge duct to a selected region in the heating chamber or for allowing the heated fluid to enter the pump from the selected region in the heating chamber through the suction chamber. In this heat treatment method of the PDP or a glass product by using the heat treatment device including the means for recirculating the heated fluid into the heating chamber, the flow direction of the heated fluid is changed periodically during the heat treatment.