To provide a highly safe heat treatment furnace in a magnetic field operating in a low leakage magnetic field with the installation cost and the manufacturing cost lower than those in prior art.

THe heat treatment furnace is adapted as follows. Adjacent magnets have magnetized directions different from each other. There are provided magnetic field generation means composed of a ring-shaped permanent magnet assembly 1 constructed into a ring by combining a plurality of permanent magnet segments each having residual magnetic flux density of 1.1 T or higher and coersive force of 1114 kA/m (14 kOe) or higher, and a heat treatment means positioned in a central cavity portion 20 of the ring-shaped permanent magnet assembly 1 and composed of, in order from the outside, a cooling means 3, a heating means 5, and a heat treatment container 6 including a heat treating holding tool 10 for holding a plurality of workpieces. The ring-shaped permanent magnet assembly has an inner diameter of 120 mm or more and an outer diameter of 120 mm or more. Further, there is substantial coincident at between the axial magnetic field center of the magnetic field generation means and the axial center of an assembly of a plurality of workpieces inserted into the heat treatment container.

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