  We discuss the two-neutron emission decay of $^{26}$O nucleus from a
viewpoint of dineutron correlations between the valence neutrons. We first
discuss how the dineutron correlation is realized both in the coordinate space
and in the momentum space by employing a three-body model. We then discuss the
decay energy spectrum as well as the angular distribution of emitted neutrons.
We show that an emission of two neutrons in back-to-back directions is
enhanced, which we interpret as a clear manifestation of the dineutron
correlation between the valence neutrons.
