Since the liquid / liquid separation occurs in hypermonotectic alloys and the liquid / liquid interface agitates mass transfer around solidifying front, it is rather difficult to achieve the aligned-rod structure. The high magnetic filed such as 10T achieved the aligned-rod structure in the Al-10at%In alloys. The in-situ observations of the monotectic solidification in the Al-10at%In alloys were performed using synchrotron radiation X-ray. Coarse and fine In rods coexisted during the unidirectional solidification without magnetic field. The local melt flow induced by the In-rich liquid / Al-rich liquid interface enhanced the mass transfer and consequently the coarse rods could continuously grow. The suppression of the local melt flow of which scale was several 10 m by the high static magnetic field resulted in the aligned-rod structure.