A heat source device comprising: a burner; an exhaust passage through which a combustion exhaust gas of the burner is discharged; a heat medium passage through which a heat medium circulates; a heat medium heat exchanger which is provided in communication with the heat medium passage in the exhaust passage and which heats the heat medium circulating through the heat medium passage by the combustion exhaust gas of the burner; a hot water supply passage through which water circulates; and a hot water supply heat exchanger which has a first heat exchanging unit, which is provided in communication with the hot water supply passage in the exhaust passage on a downstream side in relation to the heat medium heat exchanger and which heats water circulating through the hot water supply passage by the combustion exhaust gas of the burner, and a second heat exchanging unit, which is provided in communication with the heat medium passage and the hot water supply passage and which heats the water circulating through the hot water supply passage by the heat medium circulating through the heat medium passage, wherein the hot water supply heat exchanger has: a plurality of hollow plates (P1-P8) having hollow portions inside respectively, and stacked in parallel with predetermined gaps provided thereamong; a first communication section in communication with the hollow portions of the plurality of hollow plates (P1-P8); a second communication section in communication with the hollow portions of a predetermined number of hollow plates (P1-P5) from a topmost hollow plate among the plurality of hollow plates; and a third communication section in communication with the hollow portions of hollow plates (P6-P8) below the predetermined number of hollow plates (P1-P5) among the plurality of hollow plates, the first heat exchanging unit is composed of the predetermined number of hollow plates (P1-P5) from the topmost hollow plate, the first communication section, and the second communication section, the second heat exchanging unit is composed of the hollow plates (P6-P8) below the predetermined number of hollow plates (P1-P5), the first communication section, and the third communication section, and a part of the outer shell of the exhaust passage is composed of a top surface, a side surface, or a bottom surface of a hollow plate (P1, P5) constituting the first heat exchanging unit, whereby the heat medium circulating through the heat medium passage flows into the second heat exchanging unit through a passage (10 in) passing through the first heat exchanging unit, and characterized in that the heat medium flows out from the second heat exchanging unit through a passage (10 out) passing through the first heat exchanging unit. a burner; an exhaust passage through which a combustion exhaust gas of the burner is discharged; a heat medium passage through which a heat medium circulates; a heat medium heat exchanger which is provided in communication with the heat medium passage in the exhaust passage and which heats the heat medium circulating through the heat medium passage by the combustion exhaust gas of the burner; a hot water supply passage through which water circulates; and a hot water supply heat exchanger which has a first heat exchanging unit, which is provided in communication with the hot water supply passage in the exhaust passage on a downstream side in relation to the heat medium heat exchanger and which heats water circulating through the hot water supply passage by the combustion exhaust gas of the burner, and a second heat exchanging unit, which is provided in communication with the heat medium passage and the hot water supply passage and which heats the water circulating through the hot water supply passage by the heat medium circulating through the heat medium passage, wherein the hot water supply heat exchanger has: a plurality of hollow plates (P1-P8) having hollow portions inside respectively, and stacked in parallel with predetermined gaps provided thereamong; a first communication section in communication with the hollow portions of the plurality of hollow plates (P1-P8); a second communication section in communication with the hollow portions of a predetermined number of hollow plates (P1-P5) from a topmost hollow plate among the plurality of hollow plates; and a third communication section in communication with the hollow portions of hollow plates (P6-P8) below the predetermined number of hollow plates (P1-P5) among the plurality of hollow plates, the first heat exchanging unit is composed of the predetermined number of hollow plates (P1-P5) from the topmost hollow plate, the first communication section, and the second communication section, the second heat exchanging unit is composed of the hollow plates (P6-P8) below the predetermined number of hollow plates (P1-P5), the first communication section, and the third communication section, and a part of the outer shell of the exhaust passage is composed of a top surface, a side surface, or a bottom surface of a hollow plate (P1, P5) constituting the first heat exchanging unit, whereby the heat medium circulating through the heat medium passage flows into the second heat exchanging unit through a passage (10 in) passing through the first heat exchanging unit, and characterized in that the heat medium flows out from the second heat exchanging unit through a passage (10 out) passing through the first heat exchanging unit.