To provide a connecting device being easy in installation and installation releasing operation and surely maintaining an installing state.

An engaging groove 5 having the bottom edge 5' for forming a cross-sectional circular shape for surrounding the axis of an intermediate metallic pipe 1C is arranged on an outer peripheral surface of an end part 4 of a one end side metallic pipe 1A fitted to and inserted into an end part 3 of the intermediate metallic pipe 1C. A pair of cutouts 6, 6 leading to this engaging groove 5 are oppositely arranged on an outer peripheral surface of the end part 3 of the intermediate metallic pipe 1C. a pair of part pieces 8, 8 having the inside edge 7 brought into pressure contact with the bottom edge 5' of the engaging groove 5 by fitting to the cutouts 6 are oppositely arranged on a connecting metal fitting 9. Packing 10 is projected on the tip side more than the engaging groove 5 of the end part 4 of the one end side metallic pipe A on an inner peripheral surface of the end part 3 of the intermediate metallic pipe 1C.