In a multi-tube once-through boiler configured such that boiler water within water tubes is heated and evaporated to take out consumed steam, rows of water tubes arranged on the left and right of the combustion chamber are respectively connected by linear left and right upper headers provided at upper ends and linear left and right lower headers provided at lower ends, a lid body is formed on one end side facing the combustion chamber and a burner for supplying combustion gas to the combustion chamber is provided, and the burner is provided with a recovered oil supply unit for supplying recovered oil, a waste solvent supply unit for supplying a waste solvent, an injected air supply unit, a combustion air supply unit, and a control unit for controlling the supply of the recovered oil, the waste solvent, the injected air, and the combustion air.