Jinrui wa Suitai Shimashita: Yousei, Shimasu ka?

Summary

It's been several centuries since human beings declined due to the steep drop in birthrates. The most prosperous species on the Earth are "Fairies", 10 cm tall creatures with high intelligence. I became a UN arbitrator between the humans and the fairies and returned to my hometown to help my grandfather. I thought my job would not be a tough one and visited one of the fairies' habitats to say hello to them but...