  We study the end point of the first-order deconfinement phase transition in
two and 2+1 flavor QCD in the heavy quark region of the quark mass parameter
space. We determine the location of critical point at which the first-order
deconfinement phase transition changes to crossover, and calculate the
pseudo-scalar meson mass at the critical point. Performing quenched QCD
simulations on lattices with the temporal extents Nt=6 and 8, the effects of
heavy quarks are determined using the reweighting method. We adopt the hopping
parameter expansion to evaluate the quark determinants in the reweighting
factor. We estimate the truncation error of the hopping parameter expansion by
comparing the results of leading and next-to-leading order calculations, and
study the lattice spacing dependence as well as the spatial volume dependence
of the result for the critical point. The overlap problem of the reweighting
method is also examined. Our results for Nt=4 and 6 suggest that the critical
quark mass decreases as the lattice spacing decreases and increases as the
spatial volume increases.
