In order to derive a standard 12-lead electrocardiogram, ones of limb leads and chest leads which constitute a lead system for the standard 12-lead electrocardiogram using 10 electrodes are selected. Electrocardiographic waveforms corresponding to the selected ones of the limb leads and the chest leads are obtained, as measured electrocardiographic waveforms, with electrodes attached on a living body. Electrocardiographic waveforms corresponding to a remaining ones of the limb leads and the chest leads are calculated, as non-measured electrocardiographic waveforms, with a prescribed transformation matrix and the measured electrocardiographic waveforms.