The maintenance free RF ion source is expected to be one of the most promising candidates for the negative ion sources of plasma heating for fusion reactors. In order to make clear the condition for the discharge initiation of the RF source, we are developing an electromagnetic PIC model. The numerical result shows that a positive potential built-up with respect to the wall. As a result, the electron wall loss decreases and the electron density increases. The positive potential plays a key role for the suppression of wall loss and the electron confinement. The electromagnetic-PIC model developed is useful for the analysis of discharge initiation of the RF source.