  We study the quark mass dependence of the finite temperature QCD phase
transition in the heavy quark region using an effective potential defined
through the probability distribution function of the average plaquette.
Performing a simulation of SU(3) pure gauge theory, we first confirm that the
distribution function has two peaks indicating that the phase transition is of
first order in the heavy quark limit, while the first order transition turns
into a crossover as the quark mass decreases from infinity, where the mass
dependence of the distribution function is evaluated by the reweighting method
combined with the hopping parameter expansion. We determine the endpoint of the
first order transition region for N_f=1, 2, 3 and 2+1 cases. The quark mass
dependence of the latent heat is also evaluated in the first order transition
region.
