An oil boiler according to the present disclosure includes a combustion chamber in which a combustion reaction occurs, a burner including a fuel nozzle that sprays fuel of an oil type into the combustion chamber, an air nozzle that injects air into the combustion chamber, and a spark plug that ignites a mixture of the fuel and the air, an air supply pipe that guides the air supplied to the air nozzle, a duct that releases combustion gas, a heat exchanger that heats heating water by heat from the combustion reaction, and a case that receives said components. The duct includes a flue connecting adaptor to which a corrugated pipe and a flue are connected, in which the corrugated pipe is connected to an inlet of the air supply pipe into which the air is introduced, and the flue releases the combustion gas to the outside of the case.