A plate fin heat exchanger comprising: a corrugated fin having a crest portion having a crest line extending in a first direction, a trough portion having a trough line extending in the first direction, and an intermediate portion connecting the crest and trough portions (31,32) together, the crest and trough portions (31,32) being alternately arranged in a second direction orthogonal to the first direction; tube plates disposed to sandwich the corrugated fin therebetween, the tube plates being configured so as to be bonded to the corrugated fin while being in contact with respective surfaces of the crest and trough portions (31,32) of the corrugated fin, wherein the corrugated fin and the tube plates are bonded together by brazing, the intermediate portion of the corrugated fin has a plurality of protruding ribs (34, 35) protruding beyond a surface of the corrugated fin, extending between the crest and trough portions (31,32), and arranged in the first direction, and the surface of the crest portion being in contact with one of the tube plates and the surface of the trough portion being in contact with the other tube plate are configured as contact surfaces (310, 320) with no protruding rib (34, 35), characterized in that gently sloping portions (341, 351) are provided between the protruding ribs (34, 35) and the contact surfaces (310, 320) to gradually protrude from the surface of the corrugated fin with increasing distance from the contact surfaces (310, 320) of the crest and trough portions (31,32), and are continuous with the protruding ribs (34, 35), and fillets that are made of a brazing filler metal gathering during brazing are provided between the gently sloping portions (341, 351) and the tube plates. a corrugated fin having a crest portion having a crest line extending in a first direction, a trough portion having a trough line extending in the first direction, and an intermediate portion connecting the crest and trough portions (31,32) together, the crest and trough portions (31,32) being alternately arranged in a second direction orthogonal to the first direction; tube plates disposed to sandwich the corrugated fin therebetween, the tube plates being configured so as to be bonded to the corrugated fin while being in contact with respective surfaces of the crest and trough portions (31,32) of the corrugated fin, wherein the corrugated fin and the tube plates are bonded together by brazing, the intermediate portion of the corrugated fin has a plurality of protruding ribs (34, 35) protruding beyond a surface of the corrugated fin, extending between the crest and trough portions (31,32), and arranged in the first direction, and the surface of the crest portion being in contact with one of the tube plates and the surface of the trough portion being in contact with the other tube plate are configured as contact surfaces (310, 320) with no protruding rib (34, 35), characterized in that gently sloping portions (341, 351) are provided between the protruding ribs (34, 35) and the contact surfaces (310, 320) to gradually protrude from the surface of the corrugated fin with increasing distance from the contact surfaces (310, 320) of the crest and trough portions (31,32), and are continuous with the protruding ribs (34, 35), and fillets that are made of a brazing filler metal gathering during brazing are provided between the gently sloping portions (341, 351) and the tube plates.