  Considering the stresses due to the vacuum fluctuation and the electric
charge loaded over the surface of a spherical cavity, we estimate the maximum
value of the charge. Since this value is independent of the cavity size and
parameter free, it is regarded as the electric unit charge. Our result is $Q=
1.55\times 10^{-19}$ Coulomb which implies the relevant fine structure constant
$\alpha=1/145.90$.
