  We have investigated the Josephson plasma excitations in magnetic fields
parallel to the $ab$-plane in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single
crystals in a wide microwave frequency region (9.8 -- 75 GHz). It was found
that there are two kind of phase-collective modes: one increases with magnetic
fields in higher fields, approaching linear asymptotic dependence with $ck$,
which lies well above the inherent plasma frequency $\omega_p$, while the other
does not show considerable field dependence. The higher linear mode is
attributed to the Josephson plasma mode propagating along the reciprocal
lattice vector of the Josephson vortex, whereas the lower one can be ascribed
to the oscillation mode of Josephson vortices.
