  The equation of state (EoS), quark number density and susceptibility at
nonzero quark chemical potential $\mu$ are studied in lattice QCD simulations
with a clover-improved Wilson fermion of 2-flavors and RG-improved gauge
action. To access nonzero $\mu$, we employ two methods : a multi-parameter
reweighting (MPR) in $\mu$ and $\beta$ and Taylor expansion in $\mu/T$. The use
of a reduction formula for the Wilson fermion determinant enables to study the
reweighting factor in MPR explicitly and heigher-order coefficients in Taylor
expansion free from errors of noise method, although calculations are limited
to small lattice size. As a consequence, we can study the reliability of the
thermodynamical quantities through the consistency of the two methods, each of
which has different origin of the application limit.
  The thermodynamical quantities are obtained from simulations on a $8^3\times
4$ lattice with an intermediate quark mass($m_{\rm PS}/m_{\rm V}=0.8)$. The MPR
and Taylor expansion are consistent for the EoS and number density up to
$\mu/T\sim 0.8$ and for the number susceptibility up to $\mu/T \sim 0.6$. This
implies within a given statistics that the overlap problem for the MPR and
truncation error for the Taylor expansion method are negligible in these
regions.
  In order to make MPR methods work, the fluctuation of the reweighting factor
should be small. We derive the equation of the reweighting line where the
fluctuation is small, and show that the equation of the reweighting line is
consistent with the fluctuation minimum condition.
