A heartbeat signal method includes detecting first and second heartbeat signals, where the first and second heartbeat signals may be potential difference signals with respect to a reference potential. First and second average DC voltage values as voltage values of direct-current components in the first and second heartbeat signals are calculated. First and second average AC amplitude values as amplitude values of alternate-current components in the first and second heartbeat signals are calculated, and a correlation coefficient between the alternate-current component in the first heartbeat signal and the alternate-current component in the second heartbeat signal is calculated. An amplification factor is set to amplify the first or second heartbeat signal on the basis of the first and second average DC voltage values, the first and second average AC amplitude values, and the correlation coefficient. A differential heartbeat signal is generated by amplifying the first or second heartbeat signal by the amplification factor, and a difference between the first and second heartbeat signals is calculated.