To effect connection in a state that a rotation direction position is regulated when a pair of pipes to be connected are interconnected.

The connection structure of a pair of pipes 44 and 46 to be connected forms a pipe connection structure comprising flanges 56 and 48 formed at the pipes 44 and 46 to be connected; a fit-in part 50 formed at the pipe 46 to be connected; and a joint member 66 having the fit-in part 50 fitted in the pipe 44 to be connected, and nipping the flanges 56 and 48 in a state that a pair of the flanges 56 and 48 are overlapped with each other, and having a joint member 66 to keep a pair of the pipes 44 and 46 to be connected. A fit-in protrusion part 50 is formed on one of the tip surface in an axial direction of the fit-in part 50 and the inner peripheral part of the pipe 44 to be connected on the side where the fit-in part 50 is fitted in and fitting in the fit-in protrusion 54 is effected in a state that the fit in part 50 is fitted in the other. A fit-in recessed part 62 is provided to regulate the positions in a rotation direction of a pair of the pipes 44 and 46 to be connected in cooperation with the engaging protrusion 54.