  We propose a criterion for examining whether or not the uncertainty of the
outer rotation curves is sufficiently small to distinguish a Keplerian rotation
curve from flat ones. We have applied this criterion to both Keplerian and
non-Keplerian rotation curves so far obtained, and investigated their relative
fraction. We also studied the minimum extent of the dark halos indicated by
non-Keplerian rotation curves using the criterion. We have found that one
cannot rule out the possibility that a significant fraction of rotation curves
become Keplerian within 10-times the disk scale length. If the Keplerian
rotation curves so far observed trace the mass truncation, several galaxies may
have rather small halos, the extent of which is not larger than twice that of
the optical disk.
