  We present a measurement of the microlensing optical depth toward the
Galactic Bulge based on 4 years of the OGLE-II survey using Red Clump Giant
(RCG). Using 32 events we find tau=2.55_{-0.46}^{+0.57}* 10^{-6} at
(l,b)=(1.16, -2.75). Taking into account the measured gradient along the
Galactic latitude b, tau = [ (4.48+/- 2.37) + (0.78+/- 0.84)* b]* 10^{-6}, this
value is consistent with previous measurements using RCG sources and recent
theoretical predictions. We determine the microlensing parameters and select
events using a model light curve with the flux blending. We find that ~38% of
the OGLE-II events which appear to have RCG sources are actually due to much
fainter stars blended with a bright companion. We show explicitly that model
fits without blending result in similar tau estimates through partial
cancellation of contributions from higher detection efficiency, underestimated
time-scales and larger number of selected events. This approach, however, leads
to biased time-scale distributions and event rates. Consequently, microlensing
studies should carefully consider source confusion effects even for bright
stars.
