  We present line-of-sight gas sloshing first found in a cool core in a galaxy
cluster. The galaxy cluster Abell 907 is identified as a relaxed cluster owing
to its global X-ray surface brightness taken by the Chandra X-ray Observatory.
The X-ray residual image after removing the global emission of the intracluster
medium (ICM), however, shows an arc-like positive excess and a negative excess
surrounding the central positive excess in the cluster core, which in turn
indicates a disturbance of the ICM. We analyze the X-ray spectra extracted from
both regions and find that (1) the ICM temperature and the metal abundance in
the positive excess are lower and higher than those in the negative excess,
respectively, and (2) the ICM is nearly in pressure equilibrium. We also find a
slight redshift difference between the positive and the negative excesses,
which corresponds to the velocity shear of $1680^{+1300}_{-920}$ km s$^{-1}$
($1\sigma$). The X-ray residual image and the ICM properties are consistent
with those expected by line-of-sight gas sloshing. Assuming that the gas is
moving toward inverse-parallel to each other along the line-of-sight, the shear
velocity is expected to be $\sim 800$ km s$^{-1}$. The velocity field of this
level is able to provide non-thermal pressure support by $\sim 34\%$ relative
to the thermal one. The total kinetic energy inferred from the shear velocity
corresponds to $\sim 30\%$ of the bolometric luminosity of the sloshing ICM.
Abell 907 is therefore complementary to galaxy clusters in which gas sloshing
takes place in the plane of the sky, and is important for understanding gas
dynamics driven by sloshing and its influence on the heating to prevent runaway
cooling.
