It is a Japanese novel written by Fuse, and is called "That Time I Got Reincarnated as a Slime". The abbreviated name is "Tensura". It was originally serialized on the novel submission site "Shosetsuka ni Narou" on February 20, 2013, and has been published by GC Novels (Micro Magazine) since May 2014, as well as a manga based on the novel, a gaiden manga, and an anime based on the manga version. It is also being developed into a media mix, including a manga, a gaiden manga, and an anime based on the manga version.