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 prescribed transformation matrix and the measured electrocardiographic waveforms.