  The phenomenon of microlensing has successfully been used to detect
extrasolar planets. By observing characteristic, rare deviations in the
gravitational microlensing light curve one can discover that a lens is a
star--planet system. In this paper we consider an opposite case where the lens
is a single star and the source has a transiting planetary companion. We have
studied the light curve of a source star with transiting companion magnified
during microlensing event. Our model shows that in dense stellar fields, in
which blending is significant, the light drop generated by transits is greater
near the maximum of microlensing, which makes it easier to detect. We derive
the probability for the detection of a planetary transit in a microlensed
source to be of 2*10^(-6) for an individual microlensing event.
