Abstract: A ureter drain catheter is detachably connected to an extension advancing tube by insertion of a cross sectionally reduced neck at the end of the advancing tube into the proximal end of the ureter drain catheter and is clamped by a mandrin extending through the advancing tube and into the neck in the drain catheter. The design of the neck is such that, at the clamping site, the mandrin is held in place on all sides by the snadwiched neck without making contact with the inner surface of the drain catheter.