Mashita no Fudanshi-kun

Sokabe is a salaryman whose apartment is far from his workplace. He could have found a closer place but one reason stops him from moving: his feelings for his downstairs neighbor, Motoi. Going home one night with his belongings stolen, he knocks to the caretaker's door to ask for spare keys when Motoi steps in to help. Curious of the books scattered around Motoi's room, he is surprised to find that they were books on BL. Motoi catches him snooping which makes Sokabe panic and leads Motoi to assume that Sokabe is like him—a fudanshi—when in fact, Sokabe is actually gay. Now, Sokabe has to keep up the pretense of being a "fudanshi comrade" while hiding his true feelings for Motoi.

(Source: Fantasista)

Included one-shots:
Volume 1: Deaikei Hajime no Susume (The Recommendation of the Cruising Spot), Kanchigaikei Propose (The Proposal of Marriage of a Misapprehension)