The invention pertains to the field of medicine, specifically, cardiology, and may be used in functional diagnosis of cardiovascular diseases. According to the method, cardiac electromagnetic signals are recorded using the cardiac electromagnetic signals recording module, signals preliminary processing is then performed by averaging and preparing for analysis using the signals preliminary processing module； then data are analyzed and recognized using the signals transformation and analysis module, and final processing is performed using the QT, QRS, ST-T-of cardiac cycle intervals' duration calculation module, which serve as markers of myocardium's electric instability. During cardiac electromagnetic signals recording magnetic sensors of the cardiac electromagnetic signals recording module are used, isolating the magnetic component of cardiac electromagnetic signals, and magnetic field changes are recorded in one plane above the patient's chest, during data analysis and recognition using the signals transformation and analysis module single-step magnetic field maps are plotted using recorded and pre-processed heart's magnetic signals, and a "inverse problem solution" is solved, plotting single-step maps of current density vectors distribution of sources emitting the recorded magnetic signals, while every map is represented with a time interval not exceeding 4 milliseconds, and prior to final processing, using the global current density calculation module, single-step global current density values during the time interval of the whole cardiac cycle or its separate areas are calculated for each current density vectors distribution map, at final processing, calculating the duration of QT, QRS, ST-T-of cardiac cycle intervals； the time interval beginning and end are considered the point of time when the curve of global current density value change reaches or crosses the zero values line. Device for measuring QT, QRS and ST-T-of cardiac cycle intervals contains success
