Magnetocardiography (MCG) is a non-invasive and non-contact mapping technique to analyze cardiac electromagnetic activities. The SQUID (superconducting quantum interference device) system has made it possible to detect very weak cardiac magnetic signals noninvasively. In electrocardiography (ECG), the conductivity of electric current varies according to body composition, while in MCG, conductivity of magnetic field is constant regardless of body composition. Moreover, as the magnetic field detected in MCG originates not from the cardiac 'volume current' as in ECG, but from the cardiac 'primary current', the cardiac electromagnetic information of the MCG is minimally distorted. Furthermore, ECG is a record of a potential difference, so it gives only a relative value, while magnetic field strength is an absolute value. Therefore, MCG is expected to be more sensitive to minute electromagnetic abnormalities of heart disease than ECG. In this article, we discuss the usefulness of MCG in diagnosing ischemic heart diseases and arrhythmias.