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 . <EOS>