The phase formation of ferromagnetic L1₀-MnAl (τ-phase) was accelerated by annealing in magnetic field. With in-field annealing at 623 K, the magnetization of the τ-phase annealed at 15 T was a maximum of 72.2 A･m²/kg at 1.5 T, which is over 4 times larger than that annealed without an external magnetic field for the same annealing time. On the other hand, the transformation from τ-phase to β-phase was suppressed under magnetic field. The obtained bulk sample did not show magnetic anisotropy because of the ε-τ solid-state transformation. Obtained results suggested that magnetic field induced acceleration of transformation was due to the gain of Zeeman energy, which increases the driving force of the ε-τ transformation.