The Lotus Sutra, of which five Mongolian texts are available at present, is believed to have been firstly translated from Tibetan into Mongolian in the Middle period, although the original translation is missing. The oldest of the five, a Beijing xylograph printed in the 17th century, is a revision by Erdeni mergen dayicing tayiji based on the original by Chos-kyi ``od zer of the 14th century, as far as we believe the contents of its colophon. Careful comparison among the lines of those five, however, tells us that the colophon is not true at all. Moreover we can find many precious facts about Mongolian philology and historical linguistics in the lines of the Mongolian versions of the Lotus Sutra. This work is a typical case in which we find not only many mysterious facts about the compilation of Mongolian Buddhist works but also critical cues to solve the mysteries.