To provide a gas supply tube, a heat treatment furnace using the same, and a heat treatment method wherein atmospheric gas is uniformly injected over a wide area to keep uniformly the atmosphere in the het treatment furnace.

A gas injection hole 4 formed in a cylindrical section 2 is used as a through-hole formed by elongating the cylindrical section 2 axially. The gas injection hole formed in the cylindrical section is used as a substantially parallel slit shaped elongated holes located substantially parallel in its longitudinal direction with respect to an axial direction of the cylindrical section. 10r 2 or more of the gas injection holes are formed longitudinally of the cylindrical section, and 1 or 2 or more of the cylindrical sections are formed peripherally of the same.