To shorten the processing time required for heat treatment without impairing safety.

Air in an N2 purge chamber 2 adjacent to a process tube 1 is replaced with N2, to reduce the concentration of oxygen to a specified low value. With a processing gas being supplied to the process tube 1, a work piece 10 after heat treatment is carried out of the process tube 1 into the N2 purge chamber 2, while being kept at a high temperature. At this time, the processing gas is diluted at a stroke with a constant quantity of N2 with which the N2 purge chamber 2 is filled, to a concentration just short of explosion limit.