Keiri no Natsutani-san wa Gaman Dekinai

Miss Natsutani in accounting is out of everyone's league -- she works harder than anyone at work. Though she always strictly maintains her professional façade, Miss Natsutani has a secret: she's dating her coworker, Mr. Sakurano. What's worse is that she can't seem to keep her cool around him...?! Presenting a titillating office romance that blooms in the space between work life and private life!

(Source: MangaPlaza)