Abstract: The fitting for the present invention comprises a fitting main body and a retaining ring, in which the fitting main body comprises an inner core having a plurality of large diameter portions form on the outer circumferetial surface and a flange and having a hole disposed to the circumferential surface of the retaining ring for attracting the retaining ring put over the top end of the plastic pipe to the flange; and the retaining ring, upon attracting operation applied thereto, moves the plastic pipe to the flange accompanying the retaining ring to insert the inner core relatively to the top end of the plastic pipe. The inner core diametrically enlarges the top end portion of the plastic pipe and compresses the top end portion in cooperation with the retaining ring. Therefore, the said fitting can simply and reliably connect the plastic pipe.