  To discuss the dineutron correlation in the ground state, the quasi-free
neutron knockout reaction on $^6$He is investigated.In the present work, the
momentum distribution of the two emitted neutrons is calculated with the
$\alpha$~+~$n$~+~$n$ three-body model and a simple reaction model to assess the
effects of the knockout process via the $^5$He resonance and the target
dependence in the momentum distribution. A clear signature of the dineutron
correlation can be obtained by choosing the kinematical condition so the
process via the $^5$He resonance is excluded because the $^5$He resonance
drastically changes the momentum distribution. Using the proton target is
important in the quantitative discussion on the dineutron correlation by the
knockout reaction. In addition to theoretical arguments, a possible
experimental setup to determine the dineutron correlation via the quasi-free
knockout reaction is considered.
