© 2019 Japan Institute of Metals (JIM). All rights reserved. The magnetic field effect on phase formations of ferromagnetic Mn-Al（τ-phase）was evaluated. The transformation from ε-phase with hcp-structure to τ-phase was accelerated by annealing in magnetic field. With in-field annealing at 623 K, the magnetization of the sample annealed at 15 T was a maximum of 72.2 A･m2/kg at 1.5 T, which was over 4 times larger than that annealed in a zero field. Meanwhile, the precipitation of non-ferromagnetic β-phase was suppressed by in-field annealing. Magnetic field-induced acceleration of ε-τ transformation and suppression of τ-β transformation were due to the gain of Zeeman energy of τ-phase.