Development of a non-contact method for measuring the degree of corrosion is highly desired by those who are involved in the maintenance and control of steel structures. Increasingly, it is expected that application of an electromagnetic acoustic technology that does not require a coupling medium will meet this demand. Various problems have been reported regarding measuring thickness of a corroded material using the resonance method. In order to clarify these problems, it is important to make evaluations based on the resonance spectrum that corresponds to the thickness of each plate. In this paper, experiments using grooved specimens for a basic investigation were performed and a new method for wall-thinning measurement based on the resonance spectrum amplitude was proposed. In addition, examination was performed by using a numerical simulation about this method’s detection principle.