Saa Himitsu wo Hajimeyou

It is a well-known and strictly enforced fact that the company Mizuura Mai has just started working for forbids office relationships... so why is it that Katou Tomoya (also known as "Ace"), one of the top elites of the company, has decided to ask her to date him? Besides, she'd only started 3 weeks before and she barely knew him. She doesn't know where this suddenly came from! Knowing full well what it's like to be let go by a company, she doesn't want to risk it and decides to turn him down after a seemingly appropriate length of consideration time. Before she even gets there, he names her his personal secretary, to the irritation of some other office ladies who have been there longer and have, therefore, had crushes on him for longer. However, Katou tells her that he had never felt this way before and so was willing to take a risk with her, requesting that she give a relationship with him a try anyway. Mai soon finds herself acquiescing, and their secret relationship begins.

(Source: hazydaze.net)

Included one-shots:
Volume 1: Tooku, Koi wo Shiteiru.
Volume 2: Close to You, Close to Me
Volume 3: Otona Challenge?
Volume 4: Darling, Darling.
Volume 6: Neko wa Onshirazu