  The Earth's ocean mass is only 2.3 x 10^{-4} of the whole planet mass. Even
including water in the interior, it would be at most 10^{-3}-10^{-2}. Ancient
Mars may have had a similar or slightly smaller water fraction. It is important
to clarify the water delivery mechanism to rocky planets in habitable zones in
exoplanetary systems, as well as that to the Earth and Mars. Here, we consider
water delivery to planets by icy pebbles after the snowline inwardly passes the
planetary orbits and derive the water mass fraction (f_{water}) of the final
planet as a function of disk parameters and discuss the parameters that
reproduce f_{water} comparable to that inferred for the Earth and ancient Mars.
We calculate the growth of icy pebbles and their radial drift with a 1D model,
and accretion of icy pebbles onto planets, by simultaneously solving the
snowline migration and the disk dissipation, to evaluate f_{water} of the
planets. We find that f_{water} is regulated by the total mass (M_{res}) of icy
dust materials preserved in the outer disk regions at the timing (t = t_{snow})
of the snowline passage of the planetary orbit. Because M_{res} decays rapidly
after the pebble formation front reaches the disk outer edge (at t = t_{pff}),
f_{water} is sensitive to the ratio t_{snow}/t_{pff}, which is determined by
the disk parameters. We find t_{snow}/t_{pff} < 10 or > 10 is important.
Deriving an analytical formula for f_{water} that reproduces the numerical
results, we find that f_{water} of a rocky planet near 1 au is ~
10^{-4}-10^{-2}, in the disks with initial disk size ~ 30-50 au and the initial
disk mass accretion rate ~ (10^{-8}-10^{-7}) M_sun/y. Because these disks may
be median or slightly compact/massive disks, the water fraction of rocky
planets in habitable zones may be often similar to that of the Earth, if the
icy pebble accretion is responsible for the water delivery.
