  We performed deep spectropolarimetric observations of a prototypical
starburst galaxy M82 with the Subaru Telescope in order to study the kinematics
of the dust outflow. We obtained optical polarized emission-line spectra up to
$\sim$4~kpc away from the nucleus of the galaxy along three position angles
within the conical outflowing wind (superwind). The H$\alpha$ emission line in
the superwind is strongly polarized and the polarization pattern shows dust
scattering of central light sources, being consistent with the previous works.
The intensity weighted polarization degree of the H$\alpha$ line reaches
$\sim$30\%\ at maximum. There are at least two light sources at the central
region of the galaxy; one of which is located at the near-infrared nucleus and
the other resides at one of the peaks of the 3 mm radio and molecular gas
emission. The outer (> 1 kpc) dust is illuminated by the former, whereas the
inner dust is scattering the light from the latter. We investigated as well the
dust motion from the velocity field of the polarized H$\alpha$ emission line.
The dust is accelerated outward on the northwest side of the nucleus. A simple
bi-conical dust outflow model shows that the outflow velocity of the dust
reaches $\gtrsim 300-450$ km/s at $\sim 4$ kpc from the nucleus, suggesting
that some portion of the dust escapes from the gravitational potential of M82
into the intergalactic space. At some regions on the southeast side, in
particular along the position angle of 138$^\circ$, the dust has radial
velocity slower than the systemic velocity of the galaxy, apparently suggesting
inflowing motion toward the nucleus. These components are spatially consistent
with a part of the molecular gas stream, which is kinematically independent of
the outflow gas, thus the apparent inflow motion of the dust reflects the
streaming motion associated with the molecular gas stream.
