  We study the phase structure of QCD at high temperature and density by
lattice QCD simulations adopting a histogram method. The quark mass dependence
and the chemical potential dependence of the nature of phase transition are
investigated focusing on the probability distribution function (histogram). The
shape of the distribution function changes with the quark mass and chemical
potential. Through the shape of the distribution, the critical surface which
separates the first order transition and crossover regions in the heavy quark
region is determined for the (2+1)-flavor case. Moreover, we determined the
critical point at finite density for two-flavor QCD with an intermediate quark
mass, using a Gaussian approximation of the complex phase distribution of the
quark determinant. The chemical potential dependence of the critical quark mass
is also evaluated in the situation where two light quarks and many massive
quarks exist. We find that the first order transition region becomes wider with
the chemical potential in the many-flavor QCD.
