A heat exchanger that is configured to heat or cool water with a fluid, the heat exchanger comprising: a heat transfer portion in which a plurality of fluid flow paths in which a fluid flows and a plurality of water flow paths in which water flows are adjacent to each other; an upstream portion that forms an upstream space on an upstream side of the plurality of water flow paths; a distribution portion that is disposed in the upstream space and that is configured to distribute to the plurality of water flow paths water that flows into the upstream space from a water entering port, wherein the distribution portion is a plate member, wherein at least a part of the plurality of water flow paths have an opposing region that opposes the water entering port, wherein the plate member has through holes, wherein the plate member is disposed between the plurality of water flow paths and the water entering port, wherein the plate member has an opposing portion that opposes the water entering port and a non-opposing portion that does not oppose the water entering port, and wherein the through holes that are positioned at the opposing portion are smaller than the through holes that are positioned at the non-opposing portion, characterized in that a width of each of the plurality of water flow paths is less than or equal to 1 mm, the heat transfer portion has first layers and second layers alternately stacked upon each other, the water flow paths being formed in each of the first layers and the fluid flow paths being formed in each of the second layers, and the water flow paths and the fluid flow paths have linearly extending shapes and intersect each other. a heat transfer portion in which a plurality of fluid flow paths in which a fluid flows and a plurality of water flow paths in which water flows are adjacent to each other; an upstream portion that forms an upstream space on an upstream side of the plurality of water flow paths; a distribution portion that is disposed in the upstream space and that is configured to distribute to the plurality of water flow paths water that flows into the upstream space from a water entering port, wherein the distribution portion is a plate member, wherein at least a part of the plurality of water flow paths have an opposing region that opposes the water entering port, wherein the plate member has through holes, wherein the plate member is disposed between the plurality of water flow paths and the water entering port, wherein the plate member has an opposing portion that opposes the water entering port and a non-opposing portion that does not oppose the water entering port, and wherein the through holes that are positioned at the opposing portion are smaller than the through holes that are positioned at the non-opposing portion, characterized in that a width of each of the plurality of water flow paths is less than or equal to 1 mm, the heat transfer portion has first layers and second layers alternately stacked upon each other, the water flow paths being formed in each of the first layers and the fluid flow paths being formed in each of the second layers, and the water flow paths and the fluid flow paths have linearly extending shapes and intersect each other.