Abstract: A pipe connecting structure is provided to mutually join tubes, and includes a pair of flanges formed at end surfaces of a pair of pipes to be connected so as to be opposed to one another, a back-up ring interposed between the pair of flanges, a projection of the back-up ring into which the pair of flanges are fitted, and a rubber packing provided for an inner periphery of the back-up ring to be expanded and deformed in response to internal pressure of the tubes. In the pipe connecting structure, the pair of flanges are brought into pressure contact with the rubber packing and the pair of pipes are coaxially supported by clamping a housing by clamping measures.