  We study the phase structure of QCD in the $T-\mu$ plane using a histogram
method and the reweighting technique by performing phase quenched simulations
of two-flavor QCD with RG-improved gauge action and O($a$) improved Wilson
quark action. Taking the effects of the complex phase of the quark determinant
using the cumulant expansion method, we calculate the probability distribution
function of plaquette and phase-quenched determinant as a function of $T$ and
$\mu$. We discuss the order of the QCD phase transition consulting the shape of
the probability distribution function.
