This is a work that Osamu DAZAI wrote in an air-raid shelter during World War II, and is based on an illustrated book called "Mukashi-mukashi no ohanashi-yo" (literally, this is a story of a long, long time ago), which was read to children.
太宰治が第二次世界大戦の中、防空壕の中で子どもに読んで聞かせる「ムカシムカシノオ話ヨ」という絵本を元に練り上た作品。