Hidden interconnection arrangement of fence or railing and assembling method thereof

A railing device, comprising:

• a plurality of upright posts each having a flat surface, a fastening surface and two or more fastening holes spacedly formed in an upper position and a lower position of said fastening surface;
• two or more lateral beams, each having a top wall, a pressing wall and an engaging wall, wherein said top wall of each of said lateral beams has a plurality of connection openings formed at spaced intervals, said pressing wall and said engaging wall being extended from two longitudinal sides of said top wall to define a receiving chamber therebetween and a bottom opening having a width slightly smaller than a width of said top wall;
• and
• a hidden interconnection arrangement, comprising:
• two or more, elongated, engaging rails each having a flat side, a concave side opposing said flat side, and a plurality of position holes formed and extended from said concave side to said flat side at spaced intervals;
• a plurality of fasteners configured to pass through said position holes of said engaging rails and fasten to said fastening holes of said upright posts, such that said two or more engaging rails are fastened laterally across said lateral beams so as to fasten said plurality of said upright posts in parallel upright manner;
• and
• a pair of upper engaging arm and lower engaging arm integrally protruded from an inner surface of said engaging wall of each of said lateral beams, defining an engaging recess between said upper engaging arm and said lower engaging arm, wherein a width of said engaging recess is equal to or slightly wider than a width of each of said engaging rails, wherein a height of each of said upper engaging arm and said lower engaging arm is equal to or larger than a thickness of each of said engaging rails, wherein a curved bottom guider edge is formed along said lower engaging arm, such that a downward force applied to said top wall of one of said two or more lateral beams is able to press said bottom guider edge of said lower engaging arm downwards against a top edge of one of two or more engaging rails that generates a reaction force to enlarge said bottom opening of said lateral beam to allow said engaging rail to be engaged in said engaging recess and locked between said upper engaging arm and said lower engaging arm of said lateral beam while said fasteners that fastened said engaging rails to said upright posts are hidden and concealed by said lateral beams and said pressing walls of said lateral beams press against said flat surfaces of said upright posts respectively.