  A mechanism of Mott transitions in a Bose Hubbard model on a square lattice
is studied, using a variational Monte Carlo method. Besides an onsite
correlation factor, we introduce a four-body doublon-holon factor into the
trial state, which considerably improves the variational energy and can
appropriately describe a superfluid-insulator transition. Its essense consists
in binding (and unbinding) of a doublon to a holon in a finite short range,
identical with the cases of fermions. The features of this transition are
qualitatively different from those of Brinkman-Rice-type transitions.
