To prevent the occurrence of unnecessary convection, reverse flow, and stagnation of gas in a heat treatment chamber and to uniformize temperature distribution in a heat treatment chamber by a method wherein a partition plate having an approximate circular opening part is between a shelf plate and a hot blast exhaust port is arranged facing a hot blast feed port and the diameter of the opening part is specified.

An opening part 6 is arranged facing a hot blast feed port 2 between a shelf plate 7 and a hot blast exhaust port 3 in a heat treatment chamber 4. The opening part 6 approximately in a circular shape of a partition plate 5 is sized to be approximate 50-120% of the diameter of a fan 1 and when the opening is below 50% of the diameter, hot blast is centralized to a limited region along a position above the extension line of the rotary shaft of the fan 1. A reverse flow, stagnation, and convection occur to other region and when it exceeds 120%, a flow like a case that the partition plate 5 is eliminated is generated. This constitution holds temperature distribution in the heat treatment chamber 4 at a uniform value and effects the uniform feed of hot blast and improves ventilation ability of production gas during heat treatment. Thus, a product uniform in characteristics is manufactured with a high yield.