The development of dry electroencephalogram (EEG) electrodes is a topic of interest. A benefit of using dry EEG electrodes is that they do not require any skin preparation. As a result, the condition of the skin with which the dry electrodes come in contact is always different among experiments. Thus, in order to quantify the performance of EEG recordings using dry electrodes, we need an indicator of signal quality for these electrodes. In this study, we showed that signal quality changed according to the contact impedance between the skin and our dry electrodes. The results indicate that our dry electrodes could record EEG signals with high quality when the contact impedance was less than 350 kμ. However, contact between the skin and an electrode often changes even during experiments, and therefore, we contend that the contact impedance should be reduced to less than 300 kμ for safety reasons.