A heat treatment apparatus for heating an object of treatment which is a magnetic material in a magnetic field in order to order a structure of the magnetic material and to cause the magnetic material to exhibit a required magnetic property comprising a heat treatment vessel arranged to house an object of treatment, a heating device arranged to heat the object of treatment, and a magnetic field generating device arranged to impress a magnetic field onto the object of treatment; and characterized by: a holding device arranged to held the object of treatment so that the main surface of the object of treatment is in parallel with the direction of the magnetic field generated by the magnetic field generating device in the heat treatment region; a conveying device arranged to convey the holding device together with the object of treatment into the heat treatment vessel; and a cooling device provided between the magnetic field generating device and the heating device said heat treatment apparatus being characterized in that: the cooling device is arranged so that the surface temperature of the magnetic field generating device in the region in which the magnetic field generating device and the heating device are close to each other does not exceed 50 °C; the heat treatment vessel has an outer peripheral surface which extends in parallel with the horizontal direction of the entire heat treatment apparatus; the magnetic field generating device is a solenoid type DC superconducting electromagnet which is wound around the heat treatment vessel so as to surround the outer peripheral surface of the heat treatment vessel; the direction of the magnetic field generated by the magnetic field generating device in a region in which the object of treatment is heat-treated and the conveying direction of the conveying devices in which the object of treatment is conveyed into the heat treatment vessel are parallel with each other, and are in parallel with the horizontal direction of the entire heat treatment apparatus. a holding device arranged to held the object of treatment so that the main surface of the object of treatment is in parallel with the direction of the magnetic field generated by the magnetic field generating device in the heat treatment region; a conveying device arranged to convey the holding device together with the object of treatment into the heat treatment vessel; and a cooling device provided between the magnetic field generating device and the heating device said heat treatment apparatus being characterized in that: the cooling device is arranged so that the surface temperature of the magnetic field generating device in the region in which the magnetic field generating device and the heating device are close to each other does not exceed 50 °C; the heat treatment vessel has an outer peripheral surface which extends in parallel with the horizontal direction of the entire heat treatment apparatus; the magnetic field generating device is a solenoid type DC superconducting electromagnet which is wound around the heat treatment vessel so as to surround the outer peripheral surface of the heat treatment vessel; the direction of the magnetic field generated by the magnetic field generating device in a region in which the object of treatment is heat-treated and the conveying direction of the conveying devices in which the object of treatment is conveyed into the heat treatment vessel are parallel with each other, and are in parallel with the horizontal direction of the entire heat treatment apparatus. the cooling device is arranged so that the surface temperature of the magnetic field generating device in the region in which the magnetic field generating device and the heating device are close to each other does not exceed 50 °C; the heat treatment vessel has an outer peripheral surface which extends in parallel with the horizontal direction of the entire heat treatment apparatus; the magnetic field generating device is a solenoid type DC superconducting electromagnet which is wound around the heat treatment vessel so as to surround the outer peripheral surface of the heat treatment vessel; the direction of the magnetic field generated by the magnetic field generating device in a region in which the object of treatment is heat-treated and the conveying direction of the conveying devices in which the object of treatment is conveyed into the heat treatment vessel are parallel with each other, and are in parallel with the horizontal direction of the entire heat treatment apparatus.