We report neutron diffraction measurements on U(Ru_{0.96}Rh_{0.04})_{2}Si_{2} single crystal under pulsed high magnetic fields up to 30 T applied along the tetragonal c axis. The high-field experiments revealed that the field-induced phase II above 26 T corresponds to a commensurate up-up-down ferrimagnetic structure characterized by the wave vector q=(2/3,0,0) with the magnetic moments parallel to the c axis, which naturally explains the one-third magnetization plateau and the substantially changed Fermi surface in phase II. This a-axis modulated magnetic structure indicates that the phase II near the hidden order phase is closely related to the characteristic incommensurate magnetic fluctuations at Q_{1}=(0.6,0,0) in the pure system URu_{2}Si_{2}, in contrast to the pressure-induced antiferromagnetic order at Q_{0}=(1,0,0).