A heat exchanger in which flat tubes and corrugated fins are alternately arranged in parallel to constitute a core, both ends of each of the flat tubes are inserted into and fixed to a pair of tube plates, side members are disposed on both sides of the core, and both ends of each of the side members in a lengthwise direction are integrally fixed to both side walls of the tube plates in a lengthwise direction, wherein: each of the side members includes a main body part in which the side member is bent and formed into a groove shape, the cross section of which has side wall parts and a base part, along an entire length in the lengthwise direction of the main body part, wherein the main body part has substantially the same length as that of the core, each of the side members further includes stepped parts each having a tip end in the lengthwise direction formed into a stepped shape protruding toward an outside of the core, and the tip ends of the stepped parts are integrally fixed to the side walls of the pair of tube plates; and each of the stepped parts of the side members has one or more brace-like ribs integrally and protrudingly formed toward the outside of the core for connecting the main body part and the tip end of the stepped part in a slanting manner, at an intermediate part of the base part in a widthwise direction; characterized in that: each of the stepped parts of the side members includes a step surface that, except at a part provided with the brace-like rib, is formed perpendicularly to the flat tubes, and an outer surface of a bottom part of each of the pair of tube plates is seated on a respective step surface; and a tip end of the one or more brace-like ribs of the side members is configured to reach a position of an outer surface of the side wall of the tube plate. each of the side members includes a main body part in which the side member is bent and formed into a groove shape, the cross section of which has side wall parts and a base part, along an entire length in the lengthwise direction of the main body part, wherein the main body part has substantially the same length as that of the core, each of the side members further includes stepped parts each having a tip end in the lengthwise direction formed into a stepped shape protruding toward an outside of the core, and the tip ends of the stepped parts are integrally fixed to the side walls of the pair of tube plates; and each of the stepped parts of the side members has one or more brace-like ribs integrally and protrudingly formed toward the outside of the core for connecting the main body part and the tip end of the stepped part in a slanting manner, at an intermediate part of the base part in a widthwise direction; characterized in that: each of the stepped parts of the side members includes a step surface that, except at a part provided with the brace-like rib, is formed perpendicularly to the flat tubes, and an outer surface of a bottom part of each of the pair of tube plates is seated on a respective step surface; and a tip end of the one or more brace-like ribs of the side members is configured to reach a position of an outer surface of the side wall of the tube plate. each of the stepped parts of the side members includes a step surface that, except at a part provided with the brace-like rib, is formed perpendicularly to the flat tubes, and an outer surface of a bottom part of each of the pair of tube plates is seated on a respective step surface; and a tip end of the one or more brace-like ribs of the side members is configured to reach a position of an outer surface of the side wall of the tube plate.