Jinsei Saidai no Uso Tsuita

Kaoru Maeda, 35 years old, is still an aspiring artist. He works as a janitor at an art college, where he is noted only for being a nice guy. He thinks that those who aren't natural geniuses must fight in a different way. What awaits him is "the biggest lie of his life!" Maeda is suddenly labeled as a "genius artist," because of a misunderstanding by a famous creator, even though he felt conflicted about it. He even begins to live together with a woman he met by chance… After 35 years, has the man with neither luck nor opportunity finally caught his big break…!?

(Source: Big Comic Bros, translated)