  The phase transition for the $(2+1)$-dimensional spin-$S=1$ $XY$ model was
investigated numerically. Because of the boson-vortex duality, the spin
stiffness $\rho_s$ in the ordered phase and the vortex-condensate stiffness
$\rho_v$ in the disordered phase should have a close relationship. We employed
the exact diagonalization method, which yields the excitation gap directly. As
a result, we estimate the amplitude ratios $\rho_{s,v}/\Delta$ ($\Delta$: Mott
insulator gap) by means of the scaling analyses for the finite-size cluster
with $N \le 22$ spins. The ratio $\rho_s/\rho_v$ admits a quantitative measure
of deviation from selfduality.
