An insulating structure in which a side chassis part of a two-side supporting frame window chassis and a door frame in a side section are overlapping on each other, when a sliding window is closed in a two-side supporting frame window type sliding window system that supports only both sides of the glass window constituting the sliding window, wherein the insulating structure provide a supporting structure in which a roller is directly coupled to a lower glass supporting insulation bracket made of a flexible material without an aluminum chassis under the glass panel so that the roller slides along a guide rail on the door frame, and wherein the insulating structure provide a foamed insert member for blocking thermal bridge in an insertion pocket of the door frame into which the side cap made of aluminum of the side chassis portion of the sliding window is inserted when a sliding window comprising a glass support insulation bracket attached to the side of the glass panel; and a side chassis portion having a side cap made of an aluminum material provided to support the glass support insulation bracket from the inner and outer surfaces, is sliding and is closing into a door frame, characterized in that the foamed insert member for blocking thermal bridge is fixedly installed in a longitudinal direction on an inner part of the door frame with filling a gap space between the inner part of the door frame and the side cap made of an aluminum material of the side chassis portion of the sliding window, and wherein a rubber gasket is installed in a longitudinal direction on an outer surface of the foamed insert member for blocking thermal bridge in order to provide airtightness between the side caps made of aluminum and the foamed insert member for blocking thermal bridge. wherein the insulating structure provide a supporting structure in which a roller is directly coupled to a lower glass supporting insulation bracket made of a flexible material without an aluminum chassis under the glass panel so that the roller slides along a guide rail on the door frame, and wherein the insulating structure provide a foamed insert member for blocking thermal bridge in an insertion pocket of the door frame into which the side cap made of aluminum of the side chassis portion of the sliding window is inserted when a sliding window comprising a glass support insulation bracket attached to the side of the glass panel; and a side chassis portion having a side cap made of an aluminum material provided to support the glass support insulation bracket from the inner and outer surfaces, is sliding and is closing into a door frame, characterized in that the foamed insert member for blocking thermal bridge is fixedly installed in a longitudinal direction on an inner part of the door frame with filling a gap space between the inner part of the door frame and the side cap made of an aluminum material of the side chassis portion of the sliding window, and wherein a rubber gasket is installed in a longitudinal direction on an outer surface of the foamed insert member for blocking thermal bridge in order to provide airtightness between the side caps made of aluminum and the foamed insert member for blocking thermal bridge.