A plate-fin heat exchanger comprising a plate fin including: a first header flow channel; a second header flow channel; and a heat transfer flow channel connected between the first and second header flow channels (9, 10), the plate fin including: a first header region having the first header flow channel; a second header region having the second header flow channel; and a flow channel region having the heat transfer flow channel, wherein the first and second header regions (11a, 11b) are each constituted by two joined plates (16a, 16b) each having a header flow channel opening (9a, 10a), the flow channel region is constituted by a single plate in which the heat transfer flow channel is integrally formed, and the first and second header regions (11a, 11b) are integrally joined to the flow channel region, wherein the flow channel region has a heat transfer flow channel formation portion for forming the heat transfer flow channel, and the heat transfer flow channel formation portion has a first end part and a second end part opposite to the first end part, characterized in that the first end part is sandwiched between a flow channel cover provided in the first plate of the first header region and a flat surface portion of the second plate, and connected to the first header region by brazing; and the second end part is sandwiched between a flow channel cover provided in the first plate of the second header region and a flat surface portion of the second plate, and connected to the second header region by brazing. a plate fin including: a first header flow channel; a second header flow channel; and a heat transfer flow channel connected between the first and second header flow channels (9, 10), a first header flow channel; a second header flow channel; and a heat transfer flow channel connected between the first and second header flow channels (9, 10), the plate fin including: a first header region having the first header flow channel; a second header region having the second header flow channel; and a flow channel region having the heat transfer flow channel, a first header region having the first header flow channel; a second header region having the second header flow channel; and a flow channel region having the heat transfer flow channel, wherein the first and second header regions (11a, 11b) are each constituted by two joined plates (16a, 16b) each having a header flow channel opening (9a, 10a), the flow channel region is constituted by a single plate in which the heat transfer flow channel is integrally formed, and the first and second header regions (11a, 11b) are integrally joined to the flow channel region, wherein the flow channel region has a heat transfer flow channel formation portion for forming the heat transfer flow channel, and the heat transfer flow channel formation portion has a first end part and a second end part opposite to the first end part, characterized in that the first end part is sandwiched between a flow channel cover provided in the first plate of the first header region and a flat surface portion of the second plate, and connected to the first header region by brazing; and the second end part is sandwiched between a flow channel cover provided in the first plate of the second header region and a flat surface portion of the second plate, and connected to the second header region by brazing.