A heat exchanger comprising: a core having at least a plurality of first passages through which a first fluid flows and a plurality of second passages through which a second fluid flows, and at least exchanging heat between the first fluid and the second fluid, wherein the core has an inflow surface through which the first fluid flows into the core and an outflow surface through which the first fluid flows out of the core, at least the first passages each include a plurality of channels that each connect an inlet formed in the inflow surface and an outlet formed in the outflow surface together, the channels having different channel resistances, and the heat exchanger further includes: having a flow-directing member of a first type and a flow-directing member of a second type having a different configuration of the flow-directing member of the first type, the flow-directing member of the first type disposed to a side of the core where the inflow surface is located to provide a uniform distribution of a dynamic pressure of the first fluid flowing into the core to the inflow surface; and the flow-directing member of the second type is a plate-like member having many holes passing therethrough and arranged in a predetermined pattern to form an determined aperture ratio, the flow-directing member of the second type being arranged to face at least one portion of the inflow surface or of the outflow surface such that the distribution of the aperture ratio of the flow-directing member of the second type corresponds to the channel resistance distribution across the inflow surface or the outflow surface and such as to reduce a difference between flow rates of the first fluid through the channels arising from a difference in channel resistance between the channels forming each first passage of the core. a core having at least a plurality of first passages through which a first fluid flows and a plurality of second passages through which a second fluid flows, and at least exchanging heat between the first fluid and the second fluid, wherein the core has an inflow surface through which the first fluid flows into the core and an outflow surface through which the first fluid flows out of the core, at least the first passages each include a plurality of channels that each connect an inlet formed in the inflow surface and an outlet formed in the outflow surface together, the channels having different channel resistances, and the heat exchanger further includes: having a flow-directing member of a first type and a flow-directing member of a second type having a different configuration of the flow-directing member of the first type, the flow-directing member of the first type disposed to a side of the core where the inflow surface is located to provide a uniform distribution of a dynamic pressure of the first fluid flowing into the core to the inflow surface; and the flow-directing member of the second type is a plate-like member having many holes passing therethrough and arranged in a predetermined pattern to form an determined aperture ratio, the flow-directing member of the second type being arranged to face at least one portion of the inflow surface or of the outflow surface such that the distribution of the aperture ratio of the flow-directing member of the second type corresponds to the channel resistance distribution across the inflow surface or the outflow surface and such as to reduce a difference between flow rates of the first fluid through the channels arising from a difference in channel resistance between the channels forming each first passage of the core. having a flow-directing member of a first type and a flow-directing member of a second type having a different configuration of the flow-directing member of the first type, the flow-directing member of the first type disposed to a side of the core where the inflow surface is located to provide a uniform distribution of a dynamic pressure of the first fluid flowing into the core to the inflow surface; and the flow-directing member of the second type is a plate-like member having many holes passing therethrough and arranged in a predetermined pattern to form an determined aperture ratio, the flow-directing member of the second type being arranged to face at least one portion of the inflow surface or of the outflow surface such that the distribution of the aperture ratio of the flow-directing member of the second type corresponds to the channel resistance distribution across the inflow surface or the outflow surface and such as to reduce a difference between flow rates of the first fluid through the channels arising from a difference in channel resistance between the channels forming each first passage of the core.