A pipe connecting device of the type using an inner ring is provided which, through further review of the structure, has been improved so as to allow more repeated use than conventionally possible even while maintaining good seal performance. This pipe connecting device is provided with: an inner ring (3) which has an inner ring (3) comprising an inner peripheral part (3w) and an outer peripheral part (3G) which has a tapered outer peripheral expanded surface (3a) which is press-fitted into and expands the end (4C) of a tube (4); a pipe fitting main body (1) which has a cylindrical screw part (1A) with a male screw (7) attached; and a union nut (2) having a female screw (13) that screws onto the male screw (7). The inner ring (3) provided with the tube (4) is inserted into the cylindrical screw part (1A), and the tapered outer peripheral expanded surface (3a) is pressed by a pressing portion (12b) of the union nut (2) with the tube (4) interposed therebetween, wherein the pressing portion (12b) of the union nut (2) that presses the tube (4) between the tapered outer peripheral expanded surface (3a) is formed as an inclined inner peripheral surface which, relative to the axis (Y) of the cylindrical screw part (1A), is inclined in the same direction as the tapered outer peripheral expanded surface (3a).