It is aimed to reduce the size of heat exchange tubes and also to reduce pressure loss of a fluid flowing in an external flow path formed between adjacent heat exchange tubes. A first projecting portion 41 and a second projecting portion 42 of a first heat exchange tube 2A are joined to portions around an inlet 3C and outlet 3D of a second heat exchange tube 2B. A first flow path forming portion 61, a second flow path forming portion 62, and a third flow path forming portion 63 of an internal flow path 3 of each of the first heat exchange tube 2A and the second heat exchange tube 2B face a first thin portion 21A and a second thin portion 21B of the second heat exchange tube 2B or the first heat exchange tube 2A across an external flow path 4. The first flow path forming portions 61, the second flow path forming portions 62, and the third flow path forming portions 63 of the first heat exchange tube 2A and the second heat exchange tube 2B are arranged in a staggered pattern in a width direction of the heat exchange tubes 2.