A condensing boiler for purifying exhaust gas comprising: a boiler body having a combustion unit including a burner for burning fuel and a heat exchanger; an exhaust pipe having one side of a longitudinal direction which is connected to the combustion unit, and the other side of the longitudinal direction extending upwards from the boiler body to protrude to the outside, and including a carbon monoxide catalyst filter mounted inside the one side of the longitudinal direction connected to the combustion unit to filter carbon monoxide contained in the exhaust gas generated during combustion of the fuel by the combustion unit; a filtering pipe mounted to be connected to the other side of the exhaust pipe in the longitudinal direction, and including a nitrogen oxide catalyst filter detachably mounted therein to filter nitrogen oxide contained in the exhaust gas flowing from the exhaust pipe; and a filter push unit mounted on the filtering pipe so that the nitrogen oxide catalyst filter is inclined when the nitrogen oxide catalyst filter is exchanged, wherein a protrusion jaw is formed on the inner circumference of the filtering pipe having the nitrogen oxide catalyst filter to protrude in the inward direction of the filtering pipe so that an edge of one side of the nitrogen oxide catalyst filter is caught to the protrusion jaw when the nitrogen oxide catalyst filter is inserted into the filtering pipe, wherein an elastic locking member of which both ends are inserted into a locking groove formed on the inner circumference of the filtering pipe comes into contact with an edge of the other side of the nitrogen oxide catalyst filter retained to the protrusion jaw, wherein the filter push unit includes: a pushing member of which one side of the longitudinal direction gets in close contact with the edge of one side of the nitrogen oxide catalyst filter and the other side of the longitudinal direction extends to the other side of the exhaust pipe in the longitudinal direction; and an extension adjustor connected to the other side of the pushing member in the longitudinal direction and exposed to the outside through a guide long hole formed in the filtering pipe, wherein the extension adjustor includes: an extension member extending outwardly from the other side of the pushing member in the longitudinal direction; and a rotational gripping member rotatably mounted at an end portion of the extension member and having a hanging protrusion inserted into a hole formed in the outer circumference of the filtering pipe, and wherein a tension spring member is mounted in the guide long hole in such a way that one side of the tension spring member in the longitudinal direction is connected to the extension adjustor and the other side is connected to the filtering pipe. a boiler body having a combustion unit including a burner for burning fuel and a heat exchanger; an exhaust pipe having one side of a longitudinal direction which is connected to the combustion unit, and the other side of the longitudinal direction extending upwards from the boiler body to protrude to the outside, and including a carbon monoxide catalyst filter mounted inside the one side of the longitudinal direction connected to the combustion unit to filter carbon monoxide contained in the exhaust gas generated during combustion of the fuel by the combustion unit; a filtering pipe mounted to be connected to the other side of the exhaust pipe in the longitudinal direction, and including a nitrogen oxide catalyst filter detachably mounted therein to filter nitrogen oxide contained in the exhaust gas flowing from the exhaust pipe; and a filter push unit mounted on the filtering pipe so that the nitrogen oxide catalyst filter is inclined when the nitrogen oxide catalyst filter is exchanged, wherein a protrusion jaw is formed on the inner circumference of the filtering pipe having the nitrogen oxide catalyst filter to protrude in the inward direction of the filtering pipe so that an edge of one side of the nitrogen oxide catalyst filter is caught to the protrusion jaw when the nitrogen oxide catalyst filter is inserted into the filtering pipe, wherein an elastic locking member of which both ends are inserted into a locking groove formed on the inner circumference of the filtering pipe comes into contact with an edge of the other side of the nitrogen oxide catalyst filter retained to the protrusion jaw, wherein the filter push unit includes: a pushing member of which one side of the longitudinal direction gets in close contact with the edge of one side of the nitrogen oxide catalyst filter and the other side of the longitudinal direction extends to the other side of the exhaust pipe in the longitudinal direction; and an extension adjustor connected to the other side of the pushing member in the longitudinal direction and exposed to the outside through a guide long hole formed in the filtering pipe, wherein the extension adjustor includes: an extension member extending outwardly from the other side of the pushing member in the longitudinal direction; and a rotational gripping member rotatably mounted at an end portion of the extension member and having a hanging protrusion inserted into a hole formed in the outer circumference of the filtering pipe, and wherein a tension spring member is mounted in the guide long hole in such a way that one side of the tension spring member in the longitudinal direction is connected to the extension adjustor and the other side is connected to the filtering pipe.