Provided is a magnesium alloy having excellent tensile strength and good ductility. A magnesium alloy sheet is obtained by rolling a magnesium alloy having a long-period stacking ordered phase that has crystallized during casting. When the transverse section in the sheet thickness direction is observed under a scanning electron microscope substantially perpendicular to the lengthwise direction, the alloy structure is primarily a long-period stacking ordered phase and, at the observed cross section, at least two aMg phases having a thickness of 0.5 [mu]m or less in layer form are laminated with the long-period stacking ordered phase in sheet form.