  We have investigated the possible evolutional history of the water ocean on
Venus, adopting the one dimensional radiative-convective model,including the
parameters as albedo and relative humidity. Under this model, it has the
possibility that the habitable zone could include Venus. It could continue for
$\sim 1$ Gy in faint young solar flux increasing, with modest parameters such
as albedo = 0.3, relative humidity (RH=1), and $p_{n0}=10^5 $Pa. If we relax
parameters considering the 3-Dimensional calculations, the ocean could exist
there longer than $\sim$ 4.6 Gy. In such cases, we have to consider the cause
of runaway other than just solar luminosity increasing. It is important to
investigate Venus history for the coming future of Earth and observations of
exoplanets for their historical habitable zones.
