A heat exchanger for exchanging heat between a first fluid and a second fluid, comprising: a plurality of heat exchange segments each having a first flow passage and a second flow passage; a first header provided at one end of the heat exchange segments so that the first fluid is introduced into the first flow passage and the second fluid is collected from the second flow passage; and a second header provided at another end of the heat exchange segments so that the first fluid is collected from the first flow passage and the second fluid is introduced into the second flow passage, wherein each of the heat exchange segments is composed of (i) an inner tube assembly that includes two inner tubes each having the first flow passage and that is formed of the two inner tubes twisted spirally, and (ii) an exterior body accommodating the inner tube assembly so that the second flow passage is formed between an inner circumferential surface of the exterior body and an outer circumferential surface of the inner tube assembly, characterized in that a path number N indicating the number of the heat exchange segments disposed between the first header and the second header is 4 to 8, a gap width G represented by a difference ((φin / 2) - φout) between a half of an inner diameter φin of the exterior body and an outer diameter φout of the inner tube satisfies 0 < G ≤0.8 mm, and the path number N and the inner diameter φin of the exterior body satisfy one of relationships to below: N = 4 and 8.20 ≤ φin ≤ 9.50 mm N = 5 and 7.58 ≤ φin ≤ 8.90 mm N = 6 and 7.14 ≤ φin ≤ 8.50 mm N = 7 and 6.78 ≤ φin ≤ 8.20 mm N = 8 and 6.52 ≤ φin ≤ 7.90 mm a plurality of heat exchange segments each having a first flow passage and a second flow passage; a first header provided at one end of the heat exchange segments so that the first fluid is introduced into the first flow passage and the second fluid is collected from the second flow passage; and a second header provided at another end of the heat exchange segments so that the first fluid is collected from the first flow passage and the second fluid is introduced into the second flow passage, wherein each of the heat exchange segments is composed of (i) an inner tube assembly that includes two inner tubes each having the first flow passage and that is formed of the two inner tubes twisted spirally, and (ii) an exterior body accommodating the inner tube assembly so that the second flow passage is formed between an inner circumferential surface of the exterior body and an outer circumferential surface of the inner tube assembly, characterized in that a path number N indicating the number of the heat exchange segments disposed between the first header and the second header is 4 to 8, a gap width G represented by a difference ((φin / 2) - φout) between a half of an inner diameter φin of the exterior body and an outer diameter φout of the inner tube satisfies 0 < G ≤0.8 mm, and the path number N and the inner diameter φin of the exterior body satisfy one of relationships to below: N = 4 and 8.20 ≤ φin ≤ 9.50 mm N = 5 and 7.58 ≤ φin ≤ 8.90 mm N = 6 and 7.14 ≤ φin ≤ 8.50 mm N = 7 and 6.78 ≤ φin ≤ 8.20 mm N = 8 and 6.52 ≤ φin ≤ 7.90 mm N = 4 and 8.20 ≤ φin ≤ 9.50 mm N = 5 and 7.58 ≤ φin ≤ 8.90 mm N = 6 and 7.14 ≤ φin ≤ 8.50 mm N = 7 and 6.78 ≤ φin ≤ 8.20 mm N = 8 and 6.52 ≤ φin ≤ 7.90 mm