To improve molding sink without extending molding time, by arranging at least one groove along a circumferential direction in at least one of a peripheral surface and an inner circumferential surface of a gradually thickness transition part or the vicinity thereof.

A socket type resin tube joint 1 consisting of polyethylene has, for example, acceptors 11, 11 having nominal diameter 200 in its both end parts, and has a thickness part 13 in its center part through gradually thickness transition parts 12, 12 formed from the acceptors 11, 11 at both end parts along a circumferential direction. A groove 14 is arranged from the inner circumferential surfaces of the inner end parts of the acceptors 11, 11 through the whole of the inner circumferential surfaces of the gradually thickness transition parts 12, 12 and the thickness part 13. Here, a surface sink amount identified by difference between the outside diameter D1 of the opening end side of the acceptors 11, 11 having nominal diameter 200 and the outside diameter D2 of the starting point of the gradually thickness transition parts 12, 12, namely D1-D2, becomes within the width of the allowable difference of the average outside diameter of the nominal diameter 200, and the width of the allowable difference is reduced. Hereby, molding sink can be improved regardless of such a state that the resin tube joint 1 has the thickness part 13 having a great difference in thickness.