  We analyze the star formation history (SFH) of the Galactic disk by using an
infall model. Based on the observed SFH of the Galactic disk, we first
determine the timescales of the gas infall into the Galactic disk ($t_{\rm
in}$) and that of the gas consumption to form stars ($t_{\rm sf}$). Since each
of the two timescales does not prove to be determined independently from the
SFH, we first fix $t_{\rm sf}$. Then, $t_{\rm in}$ is determined so that we
minimize $\chi^2$. Consequently, we choose three parameter sets: ($t_{\rm sf}$
[Gyr], $t_{\rm in}$ [Gyr]) = (6.0, 23), (11, 12), and (15, 9.0), where we set
the Galactic age as 15 Gyr. All of the three cases predict almost identical
star formation history. Next, we test the intermittency (or variability) of the
star formation rate (SFR) along with the smooth SFH suggested from the infall
model. The large value of the $\chi^2$ statistic supports the violent
time-variation of the SFH. If we interpret the observed SFH with smooth and
variable components, the amplitude of the variable component is comparable to
the smooth component. Thus, intermittent SFH of the Galactic disk is strongly
suggested. We also examined the metallicity distribution of G-dwarfs. We found
that the true parameter set lies between ($t_{\rm sf}$ [Gyr], $t_{\rm in}$
[Gyr]) = (6, 23) and (11, 12), though we should need a more sophisticated model
including the process of metal enrichment within the Galactic halo.
