To eliminate deviation of atmospheric gas distribution in a muffle and to uniformly heat treat by injecting gas from a gas injection hole into a heat treating space while rotating a gas introducing tube by a driving mechanism, and supplying the gas into the space while changing a gas injecting direction with the lapse of time.

A muffle 1 has a heat treating space A. A gas introducing tube 3 is passed through the muffle 1 substantially in a horizontal direction perpendicular to a lengthwise direction of the muffle 1 as an axial direction and rotatably held with respect to the muffle 1. A rotary seal 4 is provided at an outer wall surface of the muffle 1 through which the tube 3 is passed to maintain hermetical sealing of the muffle 1. And, an end 3a of the tube 3 exposed to the exterior of the muffle 1 is connected to the tube 2 by a tube coupling seal 5. Further, a sprocket 6 provided at the other end 3b of the tube 3 is connected to a motor 8 via a belt 7. A plurality of gas injection holes are formed at the tube 3 at the space A side in a tube axial direction.