A story about a ronin student named Oyama, living in a small apartment, trying to survive. This series made Matsumoto famous.

(from ebookjapan):

Shota Oyama, aka "Oidon," a Kyushu boy, lives in a four-and-a-half-tatami-mat room. He has a pile of underpants in his closet, and a strange mushroom called sarmatake grows in the closet. However, even these mushrooms are valuable food in his extremely poor life.

He works part-time at a factory during the day and attends a night high school, but his failures lead to his being fired, dropping out of school, and heartbreak. Then Reiko Nishio comes to live across the hall from him. Oidon is attracted to her, but she is not the only one...