The carrier concentration dependence of the interaction between free carriers and longitudinal optical (LO) phonons of InN is studied by Raman scattering and Fourier transform infrared measurements. InN is grown on a sapphire (0001) surface by plasma-assisted molecular beam epitaxy. The carrier concentration is varied from 1.8\ifmmode×\else\texttimes\fi10181.8\ifmmode\times\else\texttimes\fi{}{10}^{18} to 1.5\ifmmode×\else\texttimes\fi1019cm\ensuremath−31.5\ifmmode\times\else\texttimes\fi{}{10}^{19}{\mathrm{cm}}^{\ensuremath{-}3} by Si doping. The infrared reflection spectra, to which the vibration in the a\ensuremath−ba\ensuremath{-}b plane contributes, reveal a linear coupling between the E1(LO){\mathrm{E}}_{1}(\mathrm{LO}) phonon and the plasma oscillation of the free carriers. From the plasma frequency the electron effective mass is estimated to be me\ensuremath⊥∗=0.085m0{m}_{e\ensuremath{\perp}}^{*}{=0.085m}_{0} for the intrinsic InN. The Raman spectra, to which the vibration along the c axis contributes, reveal that the A1(LO){\mathrm{A}}_{1}(\mathrm{LO}) phonon and free carriers couple nonlinearly, where a Fano interference between the zone-center LO phonon and the quasicontinuum electronic state along the c axis is prominent. With these results, the anisotropic electronic structure of InN is discussed.