NTT Docomo is a part of NTT Group. NTT is used to be owned by Japanese government.
In 1985 Japanese Priminister (Nakasone) took a privatization route on NTT.
As part of this process, "NTT Law" was implemented. The law requires 1/3 of NTT shares must be always held by Japanese government.
NTT Docomo is a Mobile communication wings of NTT Group. While NTT Docomo division is in greysheet, NTT Communications (Long distance and International business division) is listed in NYSE and currently traded.