According to "Kukai-sozu-den" (a biography of Kukai) written by Shinzei, he died of a disease, and it is likely that his body was cremated, according to "Shoku Nihon Koki" (Later Chronicle of Japan Continued).
真済による『空海僧都伝』によると死因は病死で、『続日本後紀』によると遺体は荼毘に付された（火葬された）ようである。