1. An apparatus employing a magnetic measurement to diagnose viable myocardium, comprising: a magnetic field distribution measurement device ( 1 ) performing a non-contact magnetic measurement on a subject's chest at a plurality of coordinates to obtain a plurality of time-series magnetic data corresponding to said plurality of coordinates, and also using said plurality of time-series magnetic data to generate time-series, magnetic field distribution data on said chest; a first arithmetic device ( 2 ) using said generated time-series, magnetic field distribution data to generate time-series, intramyocardial current density distribution data of said subject; a second arithmetic device ( 3 ) processing separately provided, tomographic, thoracic data of said subject to generate data representative of an anatomical image; and a display device ( 4 ) displaying an image of said intramyocardial, current density distribution represented by said data generated by said first arithmetic device, as superimposed on said anatomical image represented by said data generated by said second arithmetic device, thereby capable of three-dimensionally identifying a lesioned part of myocardium presenting an abnormal current density distribution or a physiologically viable part of myocardium.