PURPOSE: To provide a synthetic resin U groove which has an outstanding strength and allows easy connection and a method for manufacturing the resin.

CONSTITUTION: The subject synthetic resin U groove consists of a though-like and double-structured main system of synthetic resin with one end open, forming a U engagement part 32 starting from an end direction and with the other end where an insertion part 34 to be inserted into the engagement part. In addition, a sealing member is arranged at the depth of the engagement part 32, with which the tip of the insertion part 34 becomes engaged when the insertion part 34 is introduced into the engagement part 32. At the same time, the engagement part is prevented from slipping off the insertion part under an engagement design to allow the mutually uneven surfaces of both parts to fit into each other, when the insertion part 34 is introduced into the engagement part 32. When manufacturing the U groove, a compressed air is supplied to an area inside of a cylindrical synthetic resin material, so that the inner surfaces of both male and female dies stick to the other tightly. Thus the U groove 30 with the engagement part 32 and the insertion part 34 is formed.