  We discuss the correlation of the spin-singlet two-neutron pair, the
dineutron correlation, in the ground and excited $0^+$ states of $^8$He by
using a method to describe the dineutron correlation in neutron-rich nuclei,
the dineutron condensate wave function. The shell-model configuration and the
dineutron configuration compete with each other in the ground state of $^8$He.
We compare the ground state structure of $^8$He with that of $^6$He and suggest
that the dineutron correlation is weaker in the ground state of $^8$He than
$^6$He. We also suggest the possibility of the dineutron condensation in an
excited $0^+$ state of $^8$He. In addition, we investigate the formation
mechanism of the dineutron condensation in $^8$He by using a newly developed
framework, $\alpha$ and dineutron condensate wave function, and conclude that
the attraction from the core is essential for the formation of the dineutron
condensation.
