A transition piece assembly for a gas turbine, comprising: a transition piece including an inlet into which high-temperature combustion gas is drawn, and an outlet from which the combustion gas is discharged to a turbine inlet port; a transition piece casing disposed so as to be spaced apart from the transition piece, the transition piece casing enclosing the transition piece to form an annular interspace between an inner circumferential surface of the transition piece casing and an outer circumferential surface of the transition piece, the transition piece casing including an outer circumferential surface over which compressed air flows to cool the transition piece; a support member for supporting the transition piece casing, the support member inserted into the annular interspace and seated on the outer circumferential surface of the transition piece; and a turbine casing spaced apart from the transition piece casing and configured to enclose the transition piece casing, the turbine casing having an inner wall facing the transition piece casing; characterized in that the transition piece assembly further comprises: a guide member that includes an axially perpendicular structure, extending generally in a direction perpendicular to an axis of the transition piece casing, and configured to guide the compressed air toward the transition piece casing; wherein the guide member is fixed with respect to the inner wall of the turbine casing; and wherein the axially perpendicular structure of the guide member comprises a turbine casing guide that is fixed to the inner wall of the turbine casing and includes a guide surface facing the outer circumferential surface of the transition piece casing so that the compressed air, which flows over the outer circumferential surface of the transition piece casing and toward the turbine casing, is guided into the transition piece casing. a transition piece including an inlet into which high-temperature combustion gas is drawn, and an outlet from which the combustion gas is discharged to a turbine inlet port; a transition piece casing disposed so as to be spaced apart from the transition piece, the transition piece casing enclosing the transition piece to form an annular interspace between an inner circumferential surface of the transition piece casing and an outer circumferential surface of the transition piece, the transition piece casing including an outer circumferential surface over which compressed air flows to cool the transition piece; a support member for supporting the transition piece casing, the support member inserted into the annular interspace and seated on the outer circumferential surface of the transition piece; and a turbine casing spaced apart from the transition piece casing and configured to enclose the transition piece casing, the turbine casing having an inner wall facing the transition piece casing; characterized in that the transition piece assembly further comprises: a guide member that includes an axially perpendicular structure, extending generally in a direction perpendicular to an axis of the transition piece casing, and configured to guide the compressed air toward the transition piece casing; wherein the guide member is fixed with respect to the inner wall of the turbine casing; and wherein the axially perpendicular structure of the guide member comprises a turbine casing guide that is fixed to the inner wall of the turbine casing and includes a guide surface facing the outer circumferential surface of the transition piece casing so that the compressed air, which flows over the outer circumferential surface of the transition piece casing and toward the turbine casing, is guided into the transition piece casing. a guide member that includes an axially perpendicular structure, extending generally in a direction perpendicular to an axis of the transition piece casing, and configured to guide the compressed air toward the transition piece casing; wherein the guide member is fixed with respect to the inner wall of the turbine casing; and wherein the axially perpendicular structure of the guide member comprises a turbine casing guide that is fixed to the inner wall of the turbine casing and includes a guide surface facing the outer circumferential surface of the transition piece casing so that the compressed air, which flows over the outer circumferential surface of the transition piece casing and toward the turbine casing, is guided into the transition piece casing.