An oil boiler comprising a combustion chamber having an interior space therein in which a combustion reaction occurs; a burner including a fuel nozzle configured to spray fuel of an oil type into the interior space of the combustion chamber, an air nozzle configured to inject air into the interior space, and a spark plug configured to ignite a mixture of the fuel sprayed and the air injected; an air supply pipe configured to guide the air supplied to the air nozzle; a duct configured to release combustion gas generated in the combustion reaction; a heat exchanger configured to heat heating water by heat derived from the combustion reaction; and a case configured to receive the combustion chamber, the burner, the air supply pipe, the duct, and the heat exchanger, wherein the duct includes a flue connecting adaptor to which a corrugated pipe and a flue are connected, the corrugated pipe being connected to an inlet of the air supply pipe into which the air is introduced, and the flue being configured to release the combustion gas to the outside of the case, characterized in that: the case includes sidewalls, the flue includes an elbow(531, 531a, 531b, 531c) having an inlet and an outlet, directions of which are perpendicular to each other, the flue connecting adaptor is connected to the elbow(531, 531a, 531b, and 531c), and the elbow(531, 531a, 531b, 531c) is configured to be disposed to face one of the sidewalls by rotating the elbow(531, 531a, 531b, 531c) while the coupling with the flue connecting adaptor is maintained. a combustion chamber having an interior space therein in which a combustion reaction occurs; a burner including a fuel nozzle configured to spray fuel of an oil type into the interior space of the combustion chamber, an air nozzle configured to inject air into the interior space, and a spark plug configured to ignite a mixture of the fuel sprayed and the air injected; an air supply pipe configured to guide the air supplied to the air nozzle; a duct configured to release combustion gas generated in the combustion reaction; a heat exchanger configured to heat heating water by heat derived from the combustion reaction; and a case configured to receive the combustion chamber, the burner, the air supply pipe, the duct, and the heat exchanger, wherein the duct includes a flue connecting adaptor to which a corrugated pipe and a flue are connected, the corrugated pipe being connected to an inlet of the air supply pipe into which the air is introduced, and the flue being configured to release the combustion gas to the outside of the case, characterized in that: the case includes sidewalls, the flue includes an elbow(531, 531a, 531b, 531c) having an inlet and an outlet, directions of which are perpendicular to each other, the flue connecting adaptor is connected to the elbow(531, 531a, 531b, and 531c), and the elbow(531, 531a, 531b, 531c) is configured to be disposed to face one of the sidewalls by rotating the elbow(531, 531a, 531b, 531c) while the coupling with the flue connecting adaptor is maintained. the case includes sidewalls, the flue includes an elbow(531, 531a, 531b, 531c) having an inlet and an outlet, directions of which are perpendicular to each other, the flue connecting adaptor is connected to the elbow(531, 531a, 531b, and 531c), and the elbow(531, 531a, 531b, 531c) is configured to be disposed to face one of the sidewalls by rotating the elbow(531, 531a, 531b, 531c) while the coupling with the flue connecting adaptor is maintained.