To improve uniformity of atmosphere within a muffle body.

A muffle furnace 1 of this invention comprises a muffle body 20, heating part 30, gas supplying pipes 40, 50, transferring mechanism 60 and movable heat insulation members 70, 80. The muffle body 20 provides a processing space for applying heat treatment to a workpiece W to be transferred. The heating part 30 is arranged to enclose the upper, side and lower portions of the muffle body 20 so as to heat inside the muffle body 20. The gas supplying pipes 40, 50 pass through the side wall of the muffle body 20 in a direction crossing at a right angle with its transferring direction and are arranged side-by-side in the transferring direction. The transferring mechanism 60 transfers the workpiece within the muffle body. The heat insulation members 70, 80 are arranged to shut off a flow of heat from the heating part 30 to outside and can be moved in the transferring direction and at the same time several gas supplying pipes 40, 50 pass trough them.