Slime Taoshite 300-nen, Shiranai Uchi ni Level Max ni Nattemashita (Novel)

Slime Taoshite 300-nen, Shiranai Uchi ni Level Max ni Nattemashita (スライム倒して300年、知らないうちにレベルMAXになってました), officially in English as I've Been Killing Slimes for 300 Years and Maxed Out My Level is a Japanese light novel series written by Kisetsu Morita and illustrated by Benio.

Synopsis
Ordinary office lady Azusa Aizawa died due to overwork. For that reason, when she finds herself reborn as an immortal witch with the appearance of a 17-year-old, she decides to live an easy-going life. Her main source of income is collecting magic stones that drop off the slimes that live in the nearby forest and selling them at the nearby village's guild. After doing this for the last 300 years, she's unknowingly become a level 99 witch, the most powerful of all. [tolol]

Releases
As of May 2021, seventeen volumes have been released.

Manga
A manga adaptation of the same name by Yūsuke Shiba began serialization in  magazine under Gangan GA imprint (collaboration with GA Bunko) from June 26, 2017. The first volume was released on January 12, 2018.

Spin-off
Hira Yakunin Yatte 1500-nen, Maō no Chikara de Daijin ni Sarechaimashita (ヒラ役人やって1500年、魔王の力で大臣にされちゃいました) is the light novel spin-off by the same authors.