A heat exchanger unit comprising: a sensible heat exchanging part including a sensible heat exchange pipe(320, 320c, 320f) disposed in a sensible heat exchange area and configured to receive heating-water and allow the heating-water to flow therethrough and a sensible heat fin(330, 330b, 330c, 330f) disposed in the sensible heat exchange area and formed in a plate shape across the sensible heat exchange pipe(320, 320c, 320f) such that the sensible heat exchange pipe(320, 320c, 320f) passes through the sensible heat fin(330, 330b, 330c, 330f), the sensible heat exchange area being configured to receive sensible heat generated by a combustion reaction and to heat the heating-water; and a latent heat exchanging part including a latent heat exchange pipe(420, 420c, 420f) disposed in a latent heat exchange area and configured to receive the heating-water and allow the heating-water to flow therethrough and a latent heat fin(430, 430b, 430c, 430f) disposed in the latent heat exchange area and formed in a plate shape across the latent heat exchange pipe(420, 420c, 420f) such that the latent heat exchange pipe(420, 420c, 420f) passes through the latent heat fin(430, 430b, 430c, 430f), the latent heat exchange area being located downstream of the sensible heat exchange area based on a reference direction(D 1) that is a flow direction of combustion gas generated during the combustion reaction and configured to receive latent heat generated during a phase change of the combustion gas and to heat the heating-water, the heat exchanger unit further comprising: a housing(510, 510b, 510c, 510f) configured to surround the heat exchange areas to define the heat exchange areas therein, wherein a cross-sectional area of the heat exchange area defined on a plane perpendicular to the reference direction is referred to as a reference cross-sectional area, and based on the reference direction(D 1), the most upstream side of the fin is referred to as an inlet end, and the most downstream side is referred to as an outlet end, and characterized in that the housing(510, 510b, 510c, 510f) is provided such that the reference cross-sectional area gradually decreases from the outlet end of the sensible heat fin(330, 330b, 330c, 330f) to the inlet end of the latent heat fin(430, 430b, 430c, 430f). a sensible heat exchanging part including a sensible heat exchange pipe(320, 320c, 320f) disposed in a sensible heat exchange area and configured to receive heating-water and allow the heating-water to flow therethrough and a sensible heat fin(330, 330b, 330c, 330f) disposed in the sensible heat exchange area and formed in a plate shape across the sensible heat exchange pipe(320, 320c, 320f) such that the sensible heat exchange pipe(320, 320c, 320f) passes through the sensible heat fin(330, 330b, 330c, 330f), the sensible heat exchange area being configured to receive sensible heat generated by a combustion reaction and to heat the heating-water; and a latent heat exchanging part including a latent heat exchange pipe(420, 420c, 420f) disposed in a latent heat exchange area and configured to receive the heating-water and allow the heating-water to flow therethrough and a latent heat fin(430, 430b, 430c, 430f) disposed in the latent heat exchange area and formed in a plate shape across the latent heat exchange pipe(420, 420c, 420f) such that the latent heat exchange pipe(420, 420c, 420f) passes through the latent heat fin(430, 430b, 430c, 430f), the latent heat exchange area being located downstream of the sensible heat exchange area based on a reference direction(D 1) that is a flow direction of combustion gas generated during the combustion reaction and configured to receive latent heat generated during a phase change of the combustion gas and to heat the heating-water, the heat exchanger unit further comprising: a housing(510, 510b, 510c, 510f) configured to surround the heat exchange areas to define the heat exchange areas therein, wherein a cross-sectional area of the heat exchange area defined on a plane perpendicular to the reference direction is referred to as a reference cross-sectional area, and based on the reference direction(D 1), the most upstream side of the fin is referred to as an inlet end, and the most downstream side is referred to as an outlet end, and characterized in that the housing(510, 510b, 510c, 510f) is provided such that the reference cross-sectional area gradually decreases from the outlet end of the sensible heat fin(330, 330b, 330c, 330f) to the inlet end of the latent heat fin(430, 430b, 430c, 430f). a housing(510, 510b, 510c, 510f) configured to surround the heat exchange areas to define the heat exchange areas therein, wherein a cross-sectional area of the heat exchange area defined on a plane perpendicular to the reference direction is referred to as a reference cross-sectional area, and based on the reference direction(D 1), the most upstream side of the fin is referred to as an inlet end, and the most downstream side is referred to as an outlet end, and characterized in that the housing(510, 510b, 510c, 510f) is provided such that the reference cross-sectional area gradually decreases from the outlet end of the sensible heat fin(330, 330b, 330c, 330f) to the inlet end of the latent heat fin(430, 430b, 430c, 430f).