If a rectangular wave excitation current is used for eddy current testing (ECT) instead of a sinusoidal wave, multi-frequency testing can be directly performed, because rectangular waves contain a fundamental wave as well as harmonic waves. Furthermore, taking skin effect into account, it would be possible to identify whether the specimen has a corrosion flaw on the front surface, back surface, or flaws on both surfaces. In this study, we first investigate the method to classify these flaws on non-magnetic metal plates using rectangular wave ECT (RECT) systems with a linear amplifier. Here, we introduce an indicator that each harmonic amplitude is divided by the fundamental amplitude. The results indicate that the amount of the attenuation of the indicator, with respect to frequency, depends on the type of the flaw, and so, we can classify the type of the flaw using the indicator. Subsequently, we develop a RECT system with an inverter, and the same experiment is conducted. The results indicate that the tendency is the same as that using the RECT system with a linear amplifier, which contributes to the high efficiency, low cost, and weight reduction of the RECT.