The National Diet Library (NDL) is the sole national library of Japan, and assigns Digital Object Identifiers (DOIs) to digital surrogates of its collections and other acquired digital objects as a means for ensuring permanent accessibility to them. Those digital holdings are in our digital archiving system, the National Diet Library Digital Collections. The NDL also serves on the board of the Japan Link Centre (JaLC), which is the only organization in Japan authorized as a Registration Agency (RA) for DOI, and works together with the JaLC to promote widespread use of DOIs. This presentation explains how DOIs are assigned at the NDL, the uses to which these DOIs are being put, and indicates the issues of DOIs assigned by the NDL.