1. A method of deriving a standard 12-lead electrocardiogram, comprising: selecting ones of limb leads and chest leads which constitute a lead system for the standard 12-lead electrocardiogram using 10 electrodes; obtaining electrocardiographic waveforms corresponding to the selected ones of the limb leads and the chest leads, as measured electrocardiographic waveforms, with electrodes attached on a living body; and calculating electrocardiographic waveforms corresponding to a remaining ones of the limb leads and the chest leads, as non-measured electrocardiographic waveforms, with a single prescribed transformation matrix and the measured electrocardiographic waveforms; wherein the transformation matrix includes information indicative of a relationship between the selected ones of the chest leads and the remaining ones of the chest leads which is obtained in advance from past results of the standard 12-lead electrocardiogram using 10 electrodes; and wherein the relationship is an average relationship which is obtained from the past results of one specific person or any plurality of persons.