  Recently, OGLE team have reported clear quasar microlensing signal in
Q2237+0305. We have analyzed the microlens event of ``image C'' by using their
finely and densely sampled lightcurves. From lightcurve fitting, we can
unambiguously set the source size of $\lsim 0.98$ Einstein Ring radius as a
conservative limit. This limit corresponds to $2000 {\rm (AU)}$, if we adopt
  $M_{\rm lens} \sim 0.1M_{\odot}$ obtained by a recent statistical study of
mean mass of lens object. This gives a clear evidence for the existence of an
accretion disk in the central region of the quasar.
