We have developed a 2D3V-PIC model of the extraction region, aiming to clarify the basic extraction mechanism of H - ions from the double-ion plasma in H - negative ion sources. The result shows the same tendency of the H - ion density nH - as that observed in the experiments, i.e.,nH - in the upstream region away from the plasma meniscus (H - emitting surface) has been reduced by applying the extraction voltage. At the same time, relatively slow temporal oscillation of the electric potential compared with the electron plasma frequency has been observed in the extraction region. Results of the systematic study using a 1D3V-PIC model with the uniform magnetic field confirm the result that the electrostatic oscillation is identified to be lower hybrid wave. The effect of this oscillation on the H - transport will be studied in the future.