Sekai Ichi Hatsukoi - The World's Greatest First Love

Synopsis (en anglais)

Onodera Ritsu has just transferred from his father's publishing company to Marukawa Shoten. Assigned to the shoujo manga division, he crosses path with the person he'd least expected to meet. Takano Masamune is the editor-in-chief of the division Ritsu's been assigned to, and the two seem get off on the wrong foot right from the very start when Takano unexpectedly kisses him. However, there is more to than what meets the eye: Takano is actually Saga, Ritsu's senpai and high school love, whom Ritsu had thought to have played him for a fool by taking his affections for granted when he confessed and summarily dumped him.