A fastening member which can fix with sufficient strength siding boards whose lateral joint portions are disposed in a portion where no vertical member is provided and prevent displacement in the vertical direction, is capable of ensuring a ventilation space without using furring strips and has excellent durability, and an external wall construction structure using the same are provided. A fastening member 1 has a long fixing plate portion 11 which is fixed to at least one vertical member 21. On the front surface of the long fixing plate portion 11, a first fastening portion 12 for supporting and engaging the upper end portion in the lateral joint portions 39 of a siding board 3 and the lower end portion of the siding board thereabove, a vertical rising portion 13 for preventing displacement disposed in the lateral joint portions below the first fastening portion 12, and a second fastening portion 14 for engaging the upper end portion of the siding board in the portion other than the lateral joint portions 39 and supporting and engaging the lower end portion are provided. Each fastening portion is provided with a ventilating spacer between itself and the long fixing plate portion 11, thereby ensuring a sufficient ventilation space.