Rotation Curve Anomaly and Galactic Warp in M51
  We revisit the anomaly of rotation curve in the nearly face-on galaxy M51
that shows an apparently faster decrease of rotation velocity than the
Keplerian law in the outer disk, further showing apparent counter rotation in
the outermost HI disk. We interpret this anomaly as due to warping of the
galactic disk, and determined the warping structure of M51's disk using the
tilted-ring method, assuming that the intrinsic rotation curve is normal. It is
shown that the disk is nearly flat in the inner disk at a constant inclination
angle, but the disk suddenly bends at radius 7.5 kpc by about 27$\deg$. The
inclination angle, then, decreases monotonically outward reaching a perfect
face-on ring at 18 kpc, beyond which the disk is warped in the opposite sense
to the inner disk, resulting in apparent counter rotation.
