A heat exchanger comprising: a plurality of refrigerant tubes in which a refrigerant flows; a heat dissipation-fin in which the plurality of refrigerant tubes are inserted and through which the refrigerant and a fluid are heat-exchanged with each other; a header (50,100) coupled to at least one side of the plurality of refrigerant tubes to define a refrigerant flow space; and a guide device disposed within the header (50,100) to branch the refrigerant into a plurality of passages corresponding to the plurality of refrigerant tubes, wherein the header (50,100) comprises: a first header including a refrigerant inflow part disposed in a lower portion of the first header to allow the refrigerant to flow into the heat exchanger; and a refrigerant discharge part disposed in an upper portion of the first header to discharge the refrigerant passing through the heat exchanger; and a second header being spaced apart from the first header; the first and second headers (50, 100) extend vertically; and the guide device is disposed within the second header and is disposed on a channel closer to the refrigerant discharge part than the refrigerant inflow part among whole channels of the refrigerant flowing into the heat exchanger, wherein the guide device comprises: a partition part partitioning an inner space of the second header; characterised in that a plurality of guide parts are provided in one side of the partition part to extend in a length direction of the second header or the partition part, thereby branching the refrigerant into a plurality of flow spaces, at least one guide part of the plurality of guide parts parallely extends along a flow direction of the refrigerant, the plurality of guide parts inclinedly extend outward from a center line of the partition part. a plurality of refrigerant tubes in which a refrigerant flows; a heat dissipation-fin in which the plurality of refrigerant tubes are inserted and through which the refrigerant and a fluid are heat-exchanged with each other; a header (50,100) coupled to at least one side of the plurality of refrigerant tubes to define a refrigerant flow space; and a guide device disposed within the header (50,100) to branch the refrigerant into a plurality of passages corresponding to the plurality of refrigerant tubes, wherein the header (50,100) comprises: a first header including a refrigerant inflow part disposed in a lower portion of the first header to allow the refrigerant to flow into the heat exchanger; and a refrigerant discharge part disposed in an upper portion of the first header to discharge the refrigerant passing through the heat exchanger; and a second header being spaced apart from the first header; the first and second headers (50, 100) extend vertically; and the guide device is disposed within the second header and is disposed on a channel closer to the refrigerant discharge part than the refrigerant inflow part among whole channels of the refrigerant flowing into the heat exchanger, wherein the guide device comprises: a partition part partitioning an inner space of the second header; characterised in that a plurality of guide parts are provided in one side of the partition part to extend in a length direction of the second header or the partition part, thereby branching the refrigerant into a plurality of flow spaces, at least one guide part of the plurality of guide parts parallely extends along a flow direction of the refrigerant, the plurality of guide parts inclinedly extend outward from a center line of the partition part. a first header including a refrigerant inflow part disposed in a lower portion of the first header to allow the refrigerant to flow into the heat exchanger; and a refrigerant discharge part disposed in an upper portion of the first header to discharge the refrigerant passing through the heat exchanger; and a second header being spaced apart from the first header; the first and second headers (50, 100) extend vertically; and the guide device is disposed within the second header and is disposed on a channel closer to the refrigerant discharge part than the refrigerant inflow part among whole channels of the refrigerant flowing into the heat exchanger, wherein the guide device comprises: a partition part partitioning an inner space of the second header; characterised in that a plurality of guide parts are provided in one side of the partition part to extend in a length direction of the second header or the partition part, thereby branching the refrigerant into a plurality of flow spaces, at least one guide part of the plurality of guide parts parallely extends along a flow direction of the refrigerant, the plurality of guide parts inclinedly extend outward from a center line of the partition part. a partition part partitioning an inner space of the second header; characterised in that a plurality of guide parts are provided in one side of the partition part to extend in a length direction of the second header or the partition part, thereby branching the refrigerant into a plurality of flow spaces, at least one guide part of the plurality of guide parts parallely extends along a flow direction of the refrigerant, the plurality of guide parts inclinedly extend outward from a center line of the partition part.