A smoke tube boiler comprising: a mix chamber which includes a mixing space in which combustion gas and air are mixed, a mix chamber body having a flat shape, and a flat plate-shaped burner disposed in a horizontal direction above a combustion chamber; and a heat exchanger which includes an outer shell forming an outer wall of a tank into and from which a heat medium is introduced and discharged and which accommodates the heat medium, an upper tube plate having an end plate structure that is coupled to the outer shell and configured to form the combustion chamber, a plurality of tubes formed in a flat shape that are configured to allow combustion gas generated in the combustion chamber to flow therein and cause a heat exchange to occur between the combustion gas and the heat medium flowing outside the tubes, turbulators (240,250,280,290) coupled to an inner side of the tube and configured to induce occurrence of a turbulent flow in the flow of the combustion gas, multi-stage barriers (261,262,263) disposed between the outer shell and the tube and configured to induce a heat medium flow direction to be alternately changed between a radially inward direction and a radially outward direction, and a lower tube plate having an end plate structure that is configured to support a lower end portion of the tube, characterized in that the flat plate-shaped burner is provided across the entire area of the mixing space; and further comprising: a firing rod assembly assembled to pass through one side portion of the mix chamber to one side of the flat plate-shaped burner and configured to extend across an upper portion of the combustion chamber toward a lower side of the flat plate-shaped burner; a sealing means (160,170,180) configured to block leakage of mixed gas of the mixing space and exhaust gas of the combustion chamber to the outside through a gap between the mix chamber and the firing rod assembly, and a cooling means configured to block transfer of combustion heat generated in the combustion chamber to the sealing means (160,170,180) which is configured to seal the gap between the mix chamber and the firing rod assembly. a mix chamber which includes a mixing space in which combustion gas and air are mixed, a mix chamber body having a flat shape, and a flat plate-shaped burner disposed in a horizontal direction above a combustion chamber; and a heat exchanger which includes an outer shell forming an outer wall of a tank into and from which a heat medium is introduced and discharged and which accommodates the heat medium, an upper tube plate having an end plate structure that is coupled to the outer shell and configured to form the combustion chamber, a plurality of tubes formed in a flat shape that are configured to allow combustion gas generated in the combustion chamber to flow therein and cause a heat exchange to occur between the combustion gas and the heat medium flowing outside the tubes, turbulators (240,250,280,290) coupled to an inner side of the tube and configured to induce occurrence of a turbulent flow in the flow of the combustion gas, multi-stage barriers (261,262,263) disposed between the outer shell and the tube and configured to induce a heat medium flow direction to be alternately changed between a radially inward direction and a radially outward direction, and a lower tube plate having an end plate structure that is configured to support a lower end portion of the tube, characterized in that the flat plate-shaped burner is provided across the entire area of the mixing space; and a firing rod assembly assembled to pass through one side portion of the mix chamber to one side of the flat plate-shaped burner and configured to extend across an upper portion of the combustion chamber toward a lower side of the flat plate-shaped burner; a sealing means (160,170,180) configured to block leakage of mixed gas of the mixing space and exhaust gas of the combustion chamber to the outside through a gap between the mix chamber and the firing rod assembly, and a cooling means configured to block transfer of combustion heat generated in the combustion chamber to the sealing means (160,170,180) which is configured to seal the gap between the mix chamber and the firing rod assembly.