The crystal structure of antiferroelectric with ordered complex perovskite structure was reexamined by TEM (transmission electron microscope). Its domain-wall orientations were investigated by the spontaneous strain method. From the CBED (convergent-beam electron diffraction) patterns, its point group was determined to be orthorhombic mmm. During transition from the high-temperature cubic to low-temperature orthorhombic phase, the prototype cubic cell is monoclinically distorted. The monoclinic cell was found to be contracted along the direction of shifts. The reduced symmetry of the orthorhombic phase transformed from the prototypic phase allows the existence of ten species of domain walls, i.e. six W-walls and four -walls. The breaking of reflection symmetry during transition occurred at (1 0 0), (0 1 0), (0 1 1), and planes of the cubic prototype. These high-symmetry planes correspond to the W-walls in the ferroelastic phase. Taking into account the domain geometry, the and planes are the domain-boundaries and the others can be both and ones. The orientations of -walls calculated from the strain components were approximately , and planes at room temperature. On approaching the transition these -walls tend to orient themselves toward the direction of or domain W-walls.