There is provided a window/door system having a hidden rail structure wherein a rail structure is detachably installed at a substructure, the substructure installed where a window/door is to be installed, thereby making it very easy to replace and to maintain/repair the rail. In addition, the rail is hidden so as not to be visible to the outside when a window/door is opened/closed.That is, the present invention is to provide a window/door system having a hidden rail structure, wherein the rail structure is detachably installed at a substructure positioned where a window/door frame is to be installed, thereby making it very easy to replace and to maintain/repair the rail. Since the window/door system having a hidden rail structure according to the present invention also hides the rail so as not to be visible to the outside when a window/door is opened/closed by a sliding motion and the top of the hidden rail structure is processed so as to be flat, it not only provides a neat appearance but also makes it easy to clean. Furthermore, it has a new structure of improving air tightness against any draft and/or noise by a multileveled shielding structure.