Jinrui wa Suitai Shimashita - Yousei, Shimasu ka?

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...