  The ground and excited states of $^{8}$He were investigated with a method of
antisymmetrized molecular dynamics(AMD). We adopted effective nuclear
interactions which systematically reproduce the binding energies of $^4$He,
$^6$He and $^8$He. The ground state of $^8$He has both the $j$-$j$ coupling
feature($p_{3/2}$ closure) and the $L$-$S$ coupling feature($^4$He$+2n+2n$)
with a slight tail of dineutron at the long distance region. The theoretical
results give an indication of the $0^+_2$ state with dineutron gas-like
structure. The dineutron structure, $^4$He+$2n$+$2n$, of this state is similar
to the $3\alpha$-cluster structure of the $^{12}$C($0^+_2$) state which has
been interpreted as an $\alpha$ condensate state. Since the $^8$He($0^+_2$)
state has a significant overlap with the dineutron condensate wave function
where two dineutrons are moving in $S$ wave around the $\alpha$ core with a
dilute density, we suggest that this theoretically predicted $0^+_2$ state is a
candidate of the dineutron condensate state.
