There are provided an electrocardiograph with an extended lead function and an extended lead ECG deriving method capable of easily deriving an ECG signal of an extended lead ECG by an arithmetic operation, based on ECG signals of a standard 12-lead ECG measured by a potential detector 10. An ECG memory 12 stores the ECG signals measured as the standard 12-lead ECG by the potential detector 10. An extended lead ECG calculator 16 calculates extended lead ECGs V7-V9 from the ECG signals stored in the ECG memory 12, using coefficients α representing a relationship among leads. The extended lead ECGs V7-V9 calculated are displayed through an extended lead ECG waveform outputting device 18 on a display monitor 20.
