A radio frequency (RF) driven type hydrogen (H-) source is used as an injector of Linac4. The power transfer efficiency between the RF generator and the ion source plasma is one of the important parameters that determines the extracted H- beam current. In order to achieve efficient power transfer, it is required to match the impedance between the RF-system and plasma loading. Aiming to investigate the optimum frequency tuning, we have developed a plasma impedance prediction model. The results indicate that the amount of the power supplied to the plasma increases by tuning the frequency adequately. The model developed in the present study is useful to optimize the power transfer by the dynamic frequency tuning and can also be used for the analysis of other inductively coupled plasma (ICP) reactors.