  We prove that all holomorphic vertex operator algebras of central charge $24$
with non-trivial weight one subspaces are unitary. The main method is to use
the orbifold construction of a holomorphic VOA $V$ of central charge $24$
directly from a Niemeier lattice VOA $V_N$. We show that it is possible to
extend the unitary form for the lattice VOA $V_N$ to the holomorphic VOA $V$ by
using the orbifold construction and some information of the automorphism group
$\mathrm{Aut}(V)$.
