  Two-neutron densities obtained from microscopic wave functions of $^6$He and
$^8$He are investigated to reveal di-neutron correlations. In particular, the
comparison of the two-neutron density with the product of one-neutron densities
is useful for a quantitative discussion of di-neutron correlations. The
calculations show that the S=0 spatial two-neutron correlation increases at the
surface of $^6$He$(0^+_1)$ and $^8$He$(0^+_2)$. The enhancement is remarkable
in the $^6$He$(0^+_1)$ ground state but not as prominent in the $^8$He$(0^+_1)$
ground state. Configuration mixing of many Slater determinants is essential to
describe the di-neutron correlations. Two-neutron densities in $^{12}$C wave
functions with $\alpha$-cluster structures are also studied.
