Kocchi Muite yo Mukai-kun

The manga is about a 35-year-old office employee named Mukai-kun, who has been single for 10 years now, and still has a lingering attachment to his last ex-girlfriend, Miwako. At work, Mukai-kun starts to notice Nakatani-san, a temporary employee. Caught between an old flame he cannot let go, and a new encounter, Mukai-kun starts his "spot the difference" love game.

(Source: Anime News Network)