Ichiokunen Shugyou no Mujikaku Kensei

一億年修行の無自覚剣聖〜ただの棒だと馬鹿にされた万能の神器と、極限の修行で生み出した『剣鬼』の人格で異世界を圧倒する〜 / The Unaware Sword Saint Who Trained for 100 Million Years ~Overwhelming the Otherworld with the Almighty Divine Tool Mocked as Just a Stick and the "Sword Demon" Persona Created Through Extreme Training~ / 1억 년 수행한 무자각 검성 ~그저 막대기라고 바보 취급 당했던 만능의 신기와, 극한의 수행으로 만들어낸 '검귀'의 인격으로 이세계를 압도한다~

The only reason he is the strongest is... effort!! Ichitaro, a high school boy, has a personality that causes him to get completely absorbed in a single thing. One day, his entire class is transported to another world as heroes. The hero rank assigned to him at his destination is the worst. Deemed a useless existence, Ichitaro is locked away in a prison where time has stopped. What he did there... was training for 100 million years. By the time he left the prison, he had mastered each and every swordsmanship style, becoming the strongest hero!!