1. A magnetocardiographic diagnosis apparatus for atrial flutter and fibrillation, comprising: a magnetic field distribution measurement device ( 1 ) performing a non-contact magnetic field measurement on a subject's chest at a plurality of coordinates to obtain a plurality of time-series magnetic field data corresponding to said plurality of coordinates, and also using said plurality of time-series magnetic field 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 data representative of a three-dimensional, intramyocardial, electrical behavior 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 three-dimensional, intramyocardial, electrical behavior 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 an abnormal, intramyocardial, electrical reentry circuit.