In a firing furnace of plasma display panel, gas distribution piping and gas exhaust piping each have a circular cross section and a uniform diameter along the longitudinal direction of those pipings. The gas distribution piping has a plurality of circular openings formed in a side face thereof and the openings are constructed such that the opening becomes larger in a stepwise fashion in a direction from both end portions to the central portion of the gas distribution piping. Furthermore, the gas exhaust piping has a plurality of elliptic openings formed in a side face thereof and the openings are constructed such that the opening becomes larger in a stepwise fashion in a direction from both end portions to the central portion of the gas exhaust piping.