  The Josephson plasma resonance has been investigated in
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals in parallel magnetic fields
to the $ab$-plane. We found two resonance modes; one appears at higher
frequency in high fields above the plasma frequency $\omega_p$ at zero field
and absolute zero, and the other lies below $\omega_p$ and is observed only in
magnetic fields without considerable field dependence. Two resonance lines were
also found in numerical simulations in a single junction model with randomness
of the critical current. The higher frequency mode is attributed to the
Josephson plasma mode modified by the periodic structure of Josephson vortices,
while the lower frequency mode is interpreted as oscillations of Josephson
vortices.
