An RF discharge plasma device has been operated, to clarify the mechanism of plasma driven permeation ( PDP ) caused by the scrape-off plasma. As has been reported by us previously1, the electrons play a significant role to accelerate the PDP flux even in a low temperature plasma. Using the RF device, we measured the PDP fluxes of the deuterium as the function of bias voltage applied between the tested membrane ( iron, 0.05 mm in thickness ) and the plasma. The obtained PDP fluxes were found to strongly depend on the positive bias voltage. The Dα -emission intensity was also found to strongly depend on the positive bias voltage, and the profiles of the dependency are just consistent with the dependency of the PDP fluxes on the bias voltage. Based on the findings, it can be deduced that the acceleration of the PDP fluxes by the application of the positive bias voltage is caused by the neutral atoms of deuterium generated by the electron-impact dissociation of the deuterium molecules in the frontal region near the tested membrane.