To investigate the magnetic field effect on the synthesis reaction and phase growth in a Mn-Bi-Sn composite pellet, in-field heat treatments were performed in 5 T at 573 K for 1 to 48 hours. X-ray diffraction measurements and electron probe micro analysis revealed that MnBi phase did not appear but Mn 3 Sn, Mn 3 Sn 2 and MnSn 2 phases were detected in the pellet. In zero-field heat treatment, the phase fraction of Mn 3 Sn 2 increased and that of MnSn 2 decreased as the heat treatment time increased. In contrast, no significant change in the phase fractions of Mn 3 Sn 2 and MnSn 2 phase was observed in 5 T even if the heat treatment time was increased to 48 h. The obtained results suggested that the application of magnetic field suppressed the diffusion of Mn atoms between Mn-Sn phases.