The objective of this research is to purpose a new method of predicting the depassivation of PC steel. This is achieved by making clear the characteristics of the passivation film on PC steel in the transition state from passivation to depassivation using polished PC steel samples and electrodes immersed in an alkaline environment that simulated the interior of concrete. The new method is proposed here based on the following results. Firstly, it has been possible to judge the presence of corrosion from the change in the immersion potential of PC steel electrodes in an alkaline environment, but information about the degradation of the passivation film with time has not been obtained. Next, to clarify, it was shown that depassivation of PC steel can be predicted based on the decline in film resistance on the low frequency range in electrochemical impedance with the lapse of time. Under the experimental conditions used in this study, it was observed that depassivation could occur when the passivation film resistance was less than 2.0 MΩ・cm2. In addition, results obtained by spectroscopic ellipsometry confirmed that a correlation exists between this resistance on the low frequency range and passivation film thickness.