Sono Koi wa Tsumi desu

Overview

Okudaira-kun, the holder of three of the seven major shogi titles, is in love but unaware of it, which makes "admiration" much more awkward. Okudaira suddenly showed up in front of Arinaga, a former professional shogi player, and said, "It's been six years, but I haven't been able to forget you." He then visits Arinaga everyday, and say and does everything people called "love"... Although Okudaira seems unfriendly at first, his emotional attachment for Arinaga eventually touched the older man's heart.

Source: AniList