Four first electrodes are attached on right clavicle, the vicinity of on left clavicle, on right lowermost rib, and the position on left lowermost rib, corresponding to limb leads 12-lead electrocardiogram (ECG). Two second electrodes are attached on such positions of the living body that correspond to a lead V2 and a lead V4 of 12-lead ECG. First ECG data set correspond to leads I and II of 12-lead ECG. A second ECG data set including the leads V2 and V4. A heart vector is calculated on the first and second ECG data sets, and predetermined first lead vectors of leads I, II, V2 and V4. A third ECG data set including leads V1, V3, V5 and V6 is calculated based on the heart vector and predetermined second lead vectors of leads V1, V3, V5 and V6. A fourth ECG data set from leads III, aVR, aVL and aVF of 12-lead ECG based on the first ECG data set. The 12-lead ECG is derived based on the first to fourth ECG data sets.