A heat exchanger (100, 100A), comprising: an upstream-side tank portion which includes an upstream-side space extending in a first direction and an inflow port, the inflow port being formed on a first end portion of the upstream-side tank portion in the first direction and a cooling target fluid flowing from an outside toward the upstream-side space through the inflow port in the first direction; a core portion which includes a plurality of heat exchange tubes which are connected to a first side portion of the upstream-side tank portion in a second direction orthogonal to the first direction and are arranged in the first direction, the fluid flowing from the upstream-side space into the plurality of heat exchange tubes; and a guide member which is disposed in the upstream-side space, the guide member including a guide surface which faces the inflow port in the first direction and includes a first inclined surface inclined to extend in the second direction toward the first side portion as the first inclined surface goes away from the inflow port in the first direction, wherein the first end portion includes a first inner surface which is formed to extend in the second direction and to which the inflow port is opened, and wherein the first inner surface includes a tube-side region which is positioned between the inflow port and the first side portion, characterized in that the guide surface includes a second inclined surface which is continuously formed from a first end of the first inclined surface positioned on the first side portion side and faces the tube-side region in the first direction, the second inclined surface being inclined to extend in the first direction toward the tube-side region as the second inclined surface goes in the second direction from the first inclined surface, and the second inclined surface is formed in a concave shape in which an inclination angle (θ2) between second inclined surface and the first direction decreases as the second inclined surface goes toward the first side portion in the second direction. an upstream-side tank portion which includes an upstream-side space extending in a first direction and an inflow port, the inflow port being formed on a first end portion of the upstream-side tank portion in the first direction and a cooling target fluid flowing from an outside toward the upstream-side space through the inflow port in the first direction; a core portion which includes a plurality of heat exchange tubes which are connected to a first side portion of the upstream-side tank portion in a second direction orthogonal to the first direction and are arranged in the first direction, the fluid flowing from the upstream-side space into the plurality of heat exchange tubes; and a guide member which is disposed in the upstream-side space, the guide member including a guide surface which faces the inflow port in the first direction and includes a first inclined surface inclined to extend in the second direction toward the first side portion as the first inclined surface goes away from the inflow port in the first direction, wherein the first end portion includes a first inner surface which is formed to extend in the second direction and to which the inflow port is opened, and wherein the first inner surface includes a tube-side region which is positioned between the inflow port and the first side portion, characterized in that the guide surface includes a second inclined surface which is continuously formed from a first end of the first inclined surface positioned on the first side portion side and faces the tube-side region in the first direction, the second inclined surface being inclined to extend in the first direction toward the tube-side region as the second inclined surface goes in the second direction from the first inclined surface, and the second inclined surface is formed in a concave shape in which an inclination angle (θ2) between second inclined surface and the first direction decreases as the second inclined surface goes toward the first side portion in the second direction.