Recently, it has been known that electromagnetic radiation from electrical device leaks internal information. That is, electromagnetic radiation contains information. Especially, it causes serious problem for cryptographic modules if electromagnetic radiation contains secret information. Therefore many studies have been made on power/electromagnetic analysis attacks, which extract secret keys from cryptographic modules by analyzing waveforms of currents, voltage or electromagnetic field. The attacks assume that the waveforms should contain the information leakage in some way. However, there are few studies discussing about "mechanisms" of the information leakage via electromagnetic field. In this paper, we will give the leakage model caused by common-mode currents, which are one of dominant factors of radiation. If the common-mode currents contain the secret information, it might be possible to obtain the information from far field. In order to verify the leakage model, we implement cryptographic hardware on an FPGA board, and reveal the secret information from common-mode currents measured by using EMC measurement techniques.