A window/door system having a hidden rail structure comprising: a substructure to be open at a top and installed in a two or more parallel line arrangement where a window/door frame is to be installed, comprising: a shielding plate installed at a bottom side of the substructure, and rail holding parts formed in a single body with two inside walls of the substructure, above the shielding plate in a direction of the length of the substructure; a rail structure to be detachably secured on the rail holding parts of the substructure; a rail covering structure having a flat shaped structure, to prevent the detachable rail structure from being exposed or visible to the outside, comprising connection parts formed in a single body with an under side of the rail covering structure, so that the connection parts are connected within a connection element of the detachable rail structure; and a window/door driving structure comprising: a top plate to be connected with a chassis frame of a window/door by a fastening structure, an inside plate and an outside plate downwardly extending from the inside and outside ends of the top plate, respectively, an inside roller supporting plate and an outside roller supporting plate respectively inwardly and outwardly extending from the lower ends of the inside plate and outside plate, and rollers positioned at bottoms of the inside and outside roller supporting plates (508, 510) and secured to roll along rails of the rail structure, wherein the detachable rail structure comprises securing parts formed in a single body with the inside and outside ends of the rail structure, so that the securing parts are secured to the rail holding parts of the substructure, in that the connection element protrudes at a central part of the rail structure, so that the connection parts of the rail covering structure are inserted in the connection element, and in that the rails are respectively positioned between one of the securing parts and the connection element and between the other of the securing parts and the connection element. a substructure to be open at a top and installed in a two or more parallel line arrangement where a window/door frame is to be installed, comprising: a shielding plate installed at a bottom side of the substructure, and rail holding parts formed in a single body with two inside walls of the substructure, above the shielding plate in a direction of the length of the substructure; a rail structure to be detachably secured on the rail holding parts of the substructure; a rail covering structure having a flat shaped structure, to prevent the detachable rail structure from being exposed or visible to the outside, comprising connection parts formed in a single body with an under side of the rail covering structure, so that the connection parts are connected within a connection element of the detachable rail structure; and a window/door driving structure comprising: a top plate to be connected with a chassis frame of a window/door by a fastening structure, an inside plate and an outside plate downwardly extending from the inside and outside ends of the top plate, respectively, an inside roller supporting plate and an outside roller supporting plate respectively inwardly and outwardly extending from the lower ends of the inside plate and outside plate, and rollers positioned at bottoms of the inside and outside roller supporting plates (508, 510) and secured to roll along rails of the rail structure, wherein the detachable rail structure comprises securing parts formed in a single body with the inside and outside ends of the rail structure, so that the securing parts are secured to the rail holding parts of the substructure, in that the connection element protrudes at a central part of the rail structure, so that the connection parts of the rail covering structure are inserted in the connection element, and in that the rails are respectively positioned between one of the securing parts and the connection element and between the other of the securing parts and the connection element.