Contactless smart cards are being widely employed in electronic passports, monetary payments, access control systems, and so forth, because of their advantages such as convenience and ease of maintenance. In this paper, we present a new side-channel attack method for contactless smart cards. This method exploits the information leakage stemming from electromagnetic disturbances (EMD). We also made a convenient and low-cost EMD reader board that performs side-channel attacks on contactless smart cards. In order to demonstrate that EMDs can become another information-leakage side channel, we have carried out side-channel analysis on a commercial contactless smart card that performs 128-bit ARIA encryptions, and we have been able to successfully find all 16 bytes of the ARIA key from the target device. From our experimental results, we conclude that the proposed EMD analysis yields better results than the conventional power analysis.