  Josephson plasma resonance measurements under magnetic fields parallel to the
CuO_2 layers as functions of magnetic field, temperature, and microwave
frequency have been performed in Bi_2Sr_2CaCu_2O_{8+\delta} single crystals
with doping range being from optimal to under-doped side. The feature of the
resonance is quite unique and cannot be explained by the conventional
understandings of the Josephson plasma for H \parallel c, that requires a new
theory including coupling effect between Josephson vortex lattice and Josephson
plasma.
