  We have examined the distribution of HI and H$_2$ gases in four face-on
galaxies by using the observed dat a of CO and HI line emissions from the
literatures. We demonstrate that the gas phase transition of HI and H$_2$
occurs suddenly within a narrow range of radi us, which we call the molecular
front. We have tried to explain such phase transition in galactic scale with a
help of the phase transition theory proposed by Elmegreen. The crucial
parameters for determinating the molecular fraction $f_{\rm mol}$ are
interstellar pressure $P $, UV radiation field $U$, and metallicity $Z$, and we
have constructed a model galaxy in which $P$, $U$ a nd $Z$ obey an exponential
function of the galacto-centric radius. The model shows that the molecular
front must be a fundamental feature of galaxies which has an exponentia l disk,
and that the metallicity gradient is most crucial for the formation of the
molecular front. We have also tried to reproduce the observed molecular
fraction $f_{\rm mol}$ by giving the set of ($P$, $ U$, $Z$) observationally,
and show that the model can describe the variation of the molecular fraction
$f_ {\rm mol}$ in galaxies quite well. We discuss the implication of the
molecular front for the chemical evolution of galaxies.
