Environmental degradation of steels associated with the hydrogen embrittlement has been a serious problem worldwide. The hydrogen evolution reaction (HER) on steel has been studied by some electrochemical methods [1-3] in order to explore the hydrogen penetration behavior on steel. In the present study, the electrochemical impedance spectroscopy was applied to the investigation of the hydrogen reaction mechanisms on steel because the time constants observed in impedance spectrum can be discriminated. The reaction scheme of HER on a metal were proposed to be a multistep as described below [4]. H+ + M + e- → MHads (1) MHads + H3O+ + e- → H2 + H2O + M (2) The reaction related to the hydrogen adsorption and absorption on steel was expressed by the following reaction [4]. MHads ↔ MHabs (3) In these reactions, the M is metal, the MHad is the hydrogen adsorption, the MHab is the hydrogen absorption. The first step (Eq. (1)) and the second step (Eq. (2)) are Volmer reaction and Heyrovsky reaction, respectively. The third step is the hydrogen penetration reaction on steel. The reactions (2) and (3) are competition reactions when the MHad generated by the reaction (1). In order to investigate these competition reactions, the Faraday impedance simulations were carried out based on these reactions. When the reaction rate constant of hydrogen penetration reaction is small compared to the HER, the capacitive loop related to the electrode/solution interface was observed in the high frequency range and the capacitive loop appeared in the low frequency range. In the case of the large reaction rate constant of hydrogen penetration compared to the HER, the impedance spectrum described the capacitive loop related to the electrode/solution interface in the high frequency range and the inductive loop in the low frequency range, respectively. These results indicated that the low frequency impedance is related to the rate-determining step of the reactions (2) or (3). The electrochemical impedance measurement of steel was performed under potentiotatic polarization in order to discuss the HER and the hydrogen penetration mechanisms on steel based on the results of simulations.