© 2019 The Japan Institute of Metals and Materials. An in-field heat treatment furnace utilized for a magnet with a 50 mm room-temperature bore was made to study magnetic field effect on gas-solid reaction. Using this furnace, nitridation of Sm2Fe17 powder was performed under a 5-T magnetic field and a 0.1-MPa nitrogen gas pressure for the temperature range 623 to 743 K. Applying a magnetic field of 5 T promoted nitridation by approximately 0.6 nitrogen atoms per formula unit compared with zero-field nitridation, and almost fully nitride Sm2Fe17N2.9 was obtained at 743 K.