To provide a method for preparing a joint for a pipe with different diameter made of an olefin resin with a connecting part for a pipe with a small diameter with good dimensional accuracy.

A high density polyethylene pipe 1 is heated at a temp. being 5-30°C lower than its m.p. and the diameter is reduced by inserting it into an outer mold 2 and while the diameter-reduced part 11 is under a condition where it is inserted into the outer mold 2 as it is, it is heated at a temp. being at least m.p. of the high density polyethylene and an inner mold 6 with an outer diameter being a little larger than the inner diameter of the diameter-reduced part 11 is inserted thereinto to form a pipe connecting part 12 with a small diameter.