In electrohydrodynamic phenomena, rhythmic oscillations of current under ac electric fields were observed in dielectric liquid, C₄F₉OCH₃ (Abe et al., Jpn. J. Appl. Phys. (2010)). The oscillations of current and static pressure depended extensively on surface conditions of the electrodes, where an electric double layer on the surface of electrodes is sensitive to its smoothness. The characteristic frequency of current was defined by a crossover point on the power spectrum. This is based on non-synchronization with external ac fields above the critical frequency of current. Simulations are carried out using feedback loop including two-dimensional diffusion process. The observed crossover point on the power spectrum is reproducible qualitatively in the simulations.