An Unaware Old Man Becomes an Overpowered Streamer is a Manga/Manhwa/Manhua in english language, Comedy series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

Keita Sato, a 41-year-old single office worker, spends his days handling stress and tedious work tasks. His unique way of unwinding involves diving solo into modern-day dungeons and using ferocious monsters as batting practice! One day, his niece Hikari secretly follows him and ends up streaming his dungeon exploits...!!