Yu no Mura Harem ~Kyonyuu Haramase Onsen Taizai Ki~

The main character "Masahiko Inukai" is at the mercy of his cousin Tomomi Inukai, who he helps with her research at the university. One day, he shows up at Tomomi's laboratory to find her in a panic. Apparently, she received a notice from the government organization that funds her research, stating that if she could not produce some kind of research results in about a year, they would terminate their funding. Masahiko, thinking that Tomomi's excessive pace was her own fault, asked her if she had a plan to produce results, and she pulled out a bundle of documents. It was a document about "Chidaru Village", a village where legend has it that a village infertility problem was saved by the power of a hot spring blessed by a goddess. Until about 20 years ago, the hot spring in the village was popular as a fertility hot spring, but it is thought to have lost its efficacy and has fallen into disrepair. Tomomi points out that there are many documents that have proven that it's effective in the past, and that no one has investigated the spring's loss of power because the number of people relying on the hot spring for fertility has decreased. The protagonist, who reads Tomomi's thoughts and sighs, begins to make arrangements to conduct an investigation at the site. [Translated from DLsite]