  To study the Josephson-vortex system in the intrinsic Josephson junctions, we
have measured the flow resistance as a function of magnetic field parallel to
the $ab$ plane in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+y}$ single crystals.
Although it was known that the flow resistance increases smoothly with
increasing field, we have found novel oscillations of vortex flow resistance in
the small current regime. The period of the oscillations corresponds to the
field which is needed to add ``{\it one}'' vortex quantum per ``{\it two}''
Josephson junctions. Commensurability between the lattice spacing of Josephson
vortices along the $ab$ plane and the size of the junction is related to the
flow velocity of vortices. The results show that Josephson vortices form
triangular lattice in the state where the oscillations occur.
