  We propose a practical way of circumventing the sign problem in lattice QCD
simulations with a theta-vacuum term. This method is the reweighting method for
the QCD Lagrangian after the U_A(1) transformation. In the Lagrangian, the
P-odd mass term as a cause of the sign problem is minimized. In order to find
out a good reference system in the reweighting method, we estimate the average
reweighting factor by using the two-flavor NJL model and eventually find a good
reference system.
