A water heat exchanger comprising a first layer and a second layer that are stacked upon each other, and exchanging heat between a first fluid and a second fluid, the first layer having first flow paths formed in a plurality of rows and through which water as the first fluid flows, the second layer having second flow paths formed in a plurality of rows and through which a refrigerant as the second fluid flows, wherein when the first layer is viewed in a stacking direction of the first layer and the second layer, each first flow path extends from one end portion to another end portion of the first layer in a direction crossing a direction of arrangement of the first flow paths, when the second layer is viewed in the stacking direction, each second flow path extends from one end portion to another end portion of the second layer in a direction crossing a direction of arrangement of the second flow paths, and when the first layer is viewed in the stacking direction, the first flow paths have a meandering shape, and/or when the second layer is viewed in the stacking direction, the second flow paths have a meandering shape, characterized in that, when the first fluid is to be heated by the second fluid, the first flow paths are formed such that a flow-path cross-sectional area of a first-fluid outlet vicinity positioned in a vicinity of an outlet for the first fluid is larger than a flow-path cross-sectional area of an upstream-side portion disposed upstream of the first-fluid outlet vicinity, wherein the first flow paths are merged such that the number of flow paths at the first-fluid outlet vicinities is less than the number of flow paths at the upstream-side portions disposed upstream of the first-fluid outlet vicinities, and/or, when the first fluid is to be cooled by the second fluid, the second flow paths are formed such that a flow-path cross-sectional area of a second-fluid outlet vicinity positioned in a vicinity of an outlet for the second fluid is larger than a flow-path cross-sectional area of an upstream-side portion disposed upstream of the second-fluid outlet vicinity, wherein the second flow paths are merged such that the number of flow paths at the second-fluid outlet vicinities is less than the number of flow paths at the upstream-side portions disposed upstream of the second-fluid outlet vicinities. when the first layer is viewed in a stacking direction of the first layer and the second layer, each first flow path extends from one end portion to another end portion of the first layer in a direction crossing a direction of arrangement of the first flow paths, when the second layer is viewed in the stacking direction, each second flow path extends from one end portion to another end portion of the second layer in a direction crossing a direction of arrangement of the second flow paths, and when the first layer is viewed in the stacking direction, the first flow paths have a meandering shape, and/or when the second layer is viewed in the stacking direction, the second flow paths have a meandering shape, characterized in that, when the first fluid is to be heated by the second fluid, the first flow paths are formed such that a flow-path cross-sectional area of a first-fluid outlet vicinity positioned in a vicinity of an outlet for the first fluid is larger than a flow-path cross-sectional area of an upstream-side portion disposed upstream of the first-fluid outlet vicinity, wherein the first flow paths are merged such that the number of flow paths at the first-fluid outlet vicinities is less than the number of flow paths at the upstream-side portions disposed upstream of the first-fluid outlet vicinities, and/or, when the first fluid is to be cooled by the second fluid, the second flow paths are formed such that a flow-path cross-sectional area of a second-fluid outlet vicinity positioned in a vicinity of an outlet for the second fluid is larger than a flow-path cross-sectional area of an upstream-side portion disposed upstream of the second-fluid outlet vicinity, wherein the second flow paths are merged such that the number of flow paths at the second-fluid outlet vicinities is less than the number of flow paths at the upstream-side portions disposed upstream of the second-fluid outlet vicinities.