  The escape fraction of ionizing photons from galaxies is a crucial quantity
controlling the cosmic ionizing background radiation and the reionization.
Various estimates of this parameter can be obtained in the redshift range,
z=0--6, either from direct observations or from the observed ionizing
background intensities. We compare them homogeneously in terms of the observed
flux density ratio of ionizing ($\sim900$ \AA rest-frame) to non-ionizing
ultraviolet ($\sim1500$ \AA rest-frame) corrected for the intergalactic
absorption. The escape fraction is found to increase by an order of magnitude,
from a value less than 0.01 at $z\la1$ to about 0.1 at $z\ga4$.
