  We study the phase structure of lattice QCD with heavy quarks at finite
temperature and density by a histogram method. We determine the location of the
critical point at which the first-order deconfining transition in the
heavy-quark limit turns into a crossover at intermediate quark masses through a
change of the shape of the histogram under variation of coupling parameters. We
estimate the effect of the complex phase factor which causes the sign problem
at finite density, and show that, in heavy-quark QCD, the effect is small
around the critical point. We determine the critical surface in 2+1 flavor QCD
in the heavy-quark region at all values of the chemical potential mu including
mu=infty.
