Recently, the issue of information leakage through electromagnetic radiation has increasingly attracted attention. Using accumulated knowledge and experience, EMC-related committees have established guidelines on standardized acceptable values of EM radiation during device operation. Current electronic devices are usually designed so as to satisfy these EMC standards. However, the main aim of these standards is to reduce EM radiation that disturbs other devices, not radiation that leaks secret information. Even for EM radiation below the value specified in the guidelines, extraction of secret key information remains a possibility. In other words, unlike noise radiation in the field of EMC, there are no definite criteria regarding EM information leakage. Therefore, uncertainty remains as to whether conventional noise suppression techniques are applicable to the suppression of information leakage. This paper discusses the difference between using electromagnetic field suppression techniques for noise and for information by using a quantitative approach. Information leakage and effective countermeasures are investigated, based on a quantitative evaluation of the signal and noise components of the leakage.