We present a model of microlensing for image A of the gravitationally lensed QSO 2237+0305 for which Irwin et al. reported in 1989 an increase of the apparent luminosity by about 0.5 mag on a time scale of a few months. Our model, with the Salpeter mass function over the mass range of 0.1-1.0 M_sun_ and the transverse velocity of the lens (or observer) of 600 km s^-1^, can reproduce the reported luminosity variation if the source of the optical continuum has a radius smaller than about 2 x 10^15^ cm. This size is compatible with the accretion disk interpretation of the big ultraviolet bump in quasar spectra. Our model demonstrates a very large diversity of light curves while the source crosses individual microcaustics or clusters of microcaustics. It will take more than 100 yr before the full variety of light curves will be sampled by the observations.