An x-prism provides four triangular prisms each of which includes a hypotenuse side, a first side surface adjoining the hypotenuse side surface, a second side surface perpendicular to the first side surface, a first end surface and an opposite second end surface, the first and second end surfaces being at opposite sides of each of the triangular prisms, the triangular prisms being attached to each other with the hypotenuse side surfaces thereof facing outward. The four triangular prisms includes at least two triangular prisms with the first end surfaces thereof being coplanar with each other, and at least one triangular prism with the first end surface thereof being non-coplanar with the first end surfaces of the other triangular prisms.