A method for mapping of myocardial electric activity includes measuring electrocardiogram data or magnetocardiogram data and mapping the degree of electric activity of a myocardial surface using the electrocardiogram data or the magnetocardiogram data. A signal source of the electrocardiogram data or the magnetocardiogram data is a myocardial surface potential that is scalar quantity. The mapping uses a lead-field vector which represents the sensitivity between the myocardial surface potential and the electrocardiogram or magnetocardiogram data, and a modified lead-field vector which combines a constraint matrix with a constraint condition where no potential sources exist in a specific region.