The present invention relates to a sliding window having excellent heat insulation and air tightness. Thanks to a sealing part interposed between one side window frame and the other side window frame of a window frame, the present invention can much more efficiently prevent internal hot air from leaking out of a room, or external cold air from leaking into the room, through a gap between the one side window frame and the other side window frame. In addition, a typical guide plate does not need to be formed on the central portion of the inner peripheral surface of the window frame, and thus the portion of the window frame making contact with the guide plate also does not need to be onerously cut away in the shape of an inverted L, as in the related art, in order to prevent interference between the guide plate and the window frame.