An apparatus for heat processing a substrate includes a heat processing furnace, which has a flat inner space for accommodating the substrate, and a gas introduction unit which introduces gas supplied via a piping into the inner space of the heat processing furnace via a gas supply inlet of the heat processing furnace. The apparatus effects the heat processing on the substrate placed within a gas flow formed in the inner space. The gas introduction unit includes a first gas introduction chamber, which receives the gas supplied via the piping for reducing a flow velocity of the gas, and a second gas introduction chamber, which is in communication with the first gas introduction chamber, is formed over at least one of outer surfaces of the top furnace wall and the bottom furnace wall at the one end of the heat processing furnace, and extends in a belt-like form through an entire width of the heat processing furnace. A portion of the top furnace wall and/or bottom furnace wall, over which the second gas introduction chamber is formed, has a nozzle opening, which covers the entire width of the heat processing furnace for flowing the gas from the second gas introduction chamber into the inner space in a direction perpendicular to the top furnace wall and/or bottom furnace wall. The gas supply port and the first gas introduction chamber are preferably separated from each other by a partition which forms a portion of a cylindrical or spherical surface and has a plurality of through-holes.