Among the Twelver Shi'i doctrines, there is the idea of the ghaybah (occultation) of the twelfth Imam, which was incorporated into them as the consequence of the dispute over the successor to the Imamate after the death of the eleventh Imam, Hasan al-'Askari (d. 260/874). Even before that, however, many other Shi'i sects had claimed the ghaybah of their Imams, in order to vindicate the legitimacy of their own leaders. By reason of this fact, many Twelver Shi'ites at that time attempted to change the existing ghaybah theory into a more appropriate form for their own Imam. It seems to have taken almost two centuries for them to establish their own form of the ghaybah theory since the death of the eleventh Imam.Kitab al-Ghaybah by al-Nu'mani (d. 360/970), one of the Twelver Shi'i traditionalists in the 10th century, has place in the earlier years of this two centuries' attempt. In this paper, the author compares Nu'mani's understanding of the ghaybah with that of al-Kulayni (d. 329/941) in his Usul al-Kafi. She first shows the difference of their ideas of the ghaybah, which is to be explained as the result of their different historical settings. Secondly, she clarifies the scholastic efforts of al-Nu'mani to adapt the idea of the ghaybah to the circumstances of his contemporary Shi'ites.