2409-kai no Kanojo

, is a Japanese light novel series written by Yu Nishimura, with illustrations by Takashina @ Seijin. The series began publication on October 10, 2006 by ASCII Media Works under their Dengeki Bunko publishing imprint.

Synopsis
A world in the shape of a "tower" with innumerable layers. Each level was managed by Antroposhka, God's surrogate machine. However, after a long time, many of the Antroposhkas who have been running the hierarchical world given the proposition of creating a world where people living in the hierarchy they manage can live in "happiness" have gone mad. In a world crazy for "happiness", a boy, Sadori, goes down the tower toward the sea with his partner frog. To fulfill the only promise I made with "her". A gentle and cruel story about God and the world.