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 .