The oldest existing book was written in the beginning of the Heian period (the tenth volume of the Tanaka book (the TANAKA original copy) and dankan (fragmentary leaves of a book) of the one volume which corresponds to the same copy).
現存する最古のものは平安極初期のもの（田中本第10巻ならびにその僚巻に相当する巻1断簡）。