In a core structure of a heat exchanger, tubes and corrugated fins are alternately arranged between seat plates arranged opposite to each other with a predetermined space interposed therebetween. Both end portions of the tubes are inserted into tube holes of connection portions formed respectively in each of the top and bottom seat plates to be fixed. On the seat plates, there are provided connection portions on which are formed wall portions slanting from main body portions thereof toward the tubes and vulnerable portions formed thinner than the seat plates and in series on the wall portions and absorb thermal stress of the seat plates against the tubes by bending.