To provide a diameter expanding jig capable of expanding the diameter of the connection end part of a fluid pipe while keeping the wall thickness of the connection end part almost equal.

A diameter expanding jig 100 for the connection end part of a fluid pipe made of a synthetic resin expanding the diameter of the connection end part of the fluid pipe while keeping the wall thickness thereof almost equal is equipped with the diameter expanding part 20 inserted into the connection end part of the fluid pipe and divided into a plurality of parts and a separation means 30 for mutually separating the divided parts of the diameter expanding part 20. The slip part 50 easy to slide on the surface of the diameter expanding part 20 is formed on the surface of the diameter expanding part 20 and the divided parts of the diameter expanding part 20 inserted into the connection end part of the fluid pipe are mutually separated by the separation means 30 to expand the diameter of the connection end part of the fluid pipe.