  We present velocities of galactic outflows in seven star-forming galaxies at
$z=$5-6 with stellar masses of $M_* \sim10^{10.1}\ \rm{M_\odot}$. Although it
is challenging to observationally determine the outflow velocities, we overcome
this by using ALMA [CII]158 $\mu$m emission lines for systemic velocities and
deep Keck spectra with metal absorption lines for velocity profiles available
to date. We construct a composite Keck spectrum of the galaxies at $z=$5-6 with
the [CII]-systemic velocities, and fit outflow-line profiles to the SiII1260,
CII1335, and SiIV1394,1403 absorption lines in the composite spectrum. We
measure the maximum (90%) and central outflow velocities to be
$v_\rm{max}=700^{+180}_{-110}\ \rm{km\ s^{-1}}$ and $v_\rm{out}=
400^{+100}_{-150}\ \rm{km\ s^{-1}}$ on average, respectively, showing no
significant differences between the outflow velocities derived with the low to
high-ionization absorption lines. For $M_* \sim10^{10.1}\ \rm{M_\odot}$, we
find that the $v_\rm{max}$ value of our $z=$5-6 galaxies is 3 times higher than
those of $z\sim0$ galaxies and comparable to $z\sim2$ galaxies. Estimating the
halo circular velocity $v_\rm{cir}$ from the stellar masses and the abundance
matching results, we investigate a $v_\rm{max}$-$v_\rm{cir}$ relation.
Interestingly, $v_\rm{max}$ for galaxies with $M_*=10^{10.0-10.8}\
\rm{M_\odot}$ shows a clear positive correlation with $v_\rm{cir}$ and/or the
galaxy star formation rate over $z=$0-6 with a small scatter of $\simeq \pm
0.1$ dex, which is in good agreement with theoretical predictions. This
positive correlation suggests that the outflow velocity is physically related
to the halo circular velocity, and that the redshift evolution of $v_\rm{max}$
at fixed $M_*$ is explained by the increase in $v_\rm{cir}$ toward high
redshift.
