A block according to the present invention is a block comprising a base body part, a first protruding part, and a second protruding part, which are integrally formed with each other. The base body part has a left lower side face, a right lower side face, a left upper side face, and a right upper side face formed as respective sloping faces. The first protruding part has a lower surface thereof formed by first to third left-hand sloping faces, an upper surface thereof formed by fourth to sixth left-hand sloping faces, and a left side surface thereof formed by seventh and eighth left-hand sloping faces. The second protruding part has a lower surface thereof formed by first to third right-hand sloping faces, an upper surface thereof formed by fourth to sixth right-hand sloping faces, and a right side surface thereof formed by seventh and eighth right-hand sloping faces. The faces of the lower portion of the block are each formed with a fitting groove, and the faces of the upper portion of the block are each formed with a fitting protrusion.