To provide a two-pipe system joint capable of using a rigid pipe to an outer pipe, halving the number of parts, reducing the weight, and miniaturizing the form.

Inner O-rings 3 are provided on the outer peripheral surfaces provided on the ends of inner pipes 1 to be connected to each other and having the diameters larger than those of outer pipes 7, outer O-rings 9 are provided on the outer peripheral surfaces provided on flanges 2 so arranged as to face to each other and on the end of outer pipes 7 to be connected to each other, and having the approximately same diameter of the inner flanges 2, the inner flanges 2, the outer flanges 1 so arranged as to face to each other, and passages 15 formed between the inner flanges 2 and the outer flanges 11 are provided inside a coupling, the inner O-rings 3 and the outer O-rings 9 are pressed by the inner peripheral surface, and sealing materials 14 for forming fluid passages for fluid between facing inner flanges 2, 2 and between the outer flanges 11, 11 are formed.