The fluxon motion excited by external microwaves has been investigated in naturally stacked, intrinsic Josephson junctions of Bi 2 Sr 2 CaCu 2 O y single crystals. For mesa-shaped single-crystals in cavity resonant modes, either odd or even order subharmonic steps, in addition to fundamental steps, caused by the phase-locked oscillation of a coupled multifluxon have been successfully observed in the current-voltage characteristics, depending on the frequency of the microwaves. For mesa-shaped and as-grown single-crystals in off-resonant modes, only fundamental steps caused by the one-way flow of such a multifluxon have been observed.