To provide an in-magnetic field heat treatment apparatus which inhibits quench from occurring immediately after a refrigeration machine stops and makes contamination of a clean room caused by a refrigerant gas unlikely to occur.

A superconducting coil 2 is immersed in liquid helium 4 in a sealed refrigerant tank 3 as a solenoid type coil enclosing a through hole 5a of a vacuum insulation container 5. A refrigeration machine 6, which re-condenses a helium gas evaporated in the refrigerant tank 3, is disposed vertically downward, and two cooling ends 6b at a lower end of the refrigeration machine 6 are installed in an upper gas space in the refrigerant tank 3. Even if the refrigerant type superconducting magnet, inhibiting quench from occurring immediately after the refrigeration machine 6 stops, is used and is installed in a clean room, the structure prevents the refrigerant gas from leaking and makes the clean room unlikely to be contaminated.