A multi-tube once-through boiler configured such that both ends side of each of a plurality of water tubes communicate with each other and boiler water is supplied to each water tube, while a combustion chamber is formed inside each water tube, combustion gas from the combustion chamber is supplied outside the plurality of water tubes to heat and evaporate the boiler water within the water tubes, and consumed steam is taken out, wherein the combustion chamber has a horizontally extending cylindrical shape, each water tube has an arc shape arranged on left and right sides of the combustion chamber, a row of water tubes arranged on the left side of the combustion chamber are connected by a linear left upper header provided at an upper end and a linear left lower header provided at a lower end, a row of water tubes arranged on the right side of the combustion chamber are connected by a linear right upper header provided at an upper end and a linear right lower header provided at a lower end a door is formed on one end side facing the combustion chamber and a burner installed on an outer surface of the door and supplying combustion gas to the combustion chamber is provided, the burner is provided with a recovered oil supply unit for supplying recovered oil including at least used engine oil, a waste solvent supply unit for supplying by-product oil as a waste solvent an injected air supply unit for supplying injected air for spraying the recovered oil and the waste solvent in the burner at appropriate pressures each adjusted through a pressure control unit, a combustion air supply unit for supplying combustion air for burning the recovered oil and the waste solvent in the burner, and a control unit for controlling supply of the recovered oil, the waste solvent, the injected air, and the combustion air, and both the recovered oil and the waste solvent are used for fuel to operate. the combustion chamber has a horizontally extending cylindrical shape, each water tube has an arc shape arranged on left and right sides of the combustion chamber, a row of water tubes arranged on the left side of the combustion chamber are connected by a linear left upper header provided at an upper end and a linear left lower header provided at a lower end, a row of water tubes arranged on the right side of the combustion chamber are connected by a linear right upper header provided at an upper end and a linear right lower header provided at a lower end a door is formed on one end side facing the combustion chamber and a burner installed on an outer surface of the door and supplying combustion gas to the combustion chamber is provided, the burner is provided with a recovered oil supply unit for supplying recovered oil including at least used engine oil, a waste solvent supply unit for supplying by-product oil as a waste solvent an injected air supply unit for supplying injected air for spraying the recovered oil and the waste solvent in the burner at appropriate pressures each adjusted through a pressure control unit, a combustion air supply unit for supplying combustion air for burning the recovered oil and the waste solvent in the burner, and a control unit for controlling supply of the recovered oil, the waste solvent, the injected air, and the combustion air, and both the recovered oil and the waste solvent are used for fuel to operate.