It's a Little Late, but I've Fallen in Love with My Childhood Friend.

A manga adaptation of the novel written by Maruto Fumiaki. The artist for this adaptation is Yom, the mangaka of Ganbare, Douki-chan.

Additional Links:

• Alternative Official Raws: niconico Manga, X (Formerly known as Twitter.)
• Buy the collected volumes from Melonbooks or TORANOANA: 1, 2