  Binder-cumulant analysis of the critical point in the heavy-quark region of
QCD is performed by Monte-Carlo simulations with the hopping-parameter
expansion at $N_t=6$. We extend our previous analysis at $N_t=4$ to finer
lattices and perform high-precision analyses on large spatial volumes up to the
aspect ratio $LT=N_s/N_t=18$. Higher order terms in the hopping-parameter
expansion are incorporated effectively up to 14th order. The numerical results
show that the violation of the finite-size scaling becomes more prominent on
the finer lattice at a given aspect ratio.
