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 , 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 are each constituted by two joined plates each having a header flow channel opening , 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 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 , 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 , 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 are each constituted by two joined plates each having a header flow channel opening , 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 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. <EOS>