The Buddhist “mummies” appeared in China during the 4th century and during the 11th century in Japan, with the exception of the corpse of the monk Kukai, founder of Shingon [in Japan], the esoteric school of Buddhism at the 9th century. The monk Kukai (posthumous name Kobo Daishi) is the most famous case. He would have entered in samadhi, at the end of his life, at the Koya mountain in the south of Osaka (Japan) at the beginning of the 9th century. His is the legendary model which the monk-ascetics of the Edo period were to follow from the 12th til19th century whose mummies were found in the North of Japan. The Japanese tradition reports that Kukai, at the time to of his death, announced to his disciples that he was going to enter Samadhi to leave there only at the time of the coming of the future Buddha Maitreya. At the end 49 days (7 times 7), his disciples opened the sarcophagus and noted that he was “as if living sat in meditation”; 70 years later, another eminent monk went up on imperial order to the top of MT Koya to open the mausoleum once again and found the body intact. He left after having cut the hair of Kukai (which had continued to grow) and having changed his clothes. The door of the mausoleum was not reopened except every fifty years by the Archbishop of Koya san to cut the nails and the hair and to change his clothes for him which will then be used to manufacture amulets for the faithful. …

The Tetsumonkai saint ascetics of the Chuzen temple, Chûkai of the Dainichibô temple, those of the Kaikoji temple or the 18 others, all chose this self-mummification at the end of their life, to give to the world the merits acquired during the course of their life because the population suffered from epidemics. The monk Chukai began his life of asceticism by offering his left eye to the god-dragon to benefit Tokyo which suffered at the time from an epidemic of pox. His altruistic gesture reproduced that of the future Sakyamuni Buddha which in one his former lives, offered his life to the tiger so that it could have milk in sufficiency to be able to nurse its young. During the feudal times, the epidemics were thought to be the manifestation of demons. It seems that the belief in the continuity of the supernatural capacities of the Saints remains even beyond death, through the relics. There is thus a survival of the Saints beyond death itself. I make a point of specifying that the presence of only one relic is equivalent to the presence of an alive Buddha in flesh and bone! Not only do the relics have all the capacities of the late one, but they connect the world of the living to the invisible world.

http://members.shaw.ca/shugendo/mummies.html