A Regularity Index (RI) measurement/notification apparatus includes: a displaying section configured to display waveform data of an intracardiac electrocardiogram a waveform data acquiring section configured to acquire waveform data in a preset waveform data acquisition time period from an intracardiac electrocardiogram during atrial fibrillation an RI calculating section configured to calculate an RI value based on the waveform data acquired by the waveform data acquiring section and a notifying section configured to, in a case where the RI value calculated by the RI calculating section exceeds a preset threshold, notify that the value exceeds the threshold.
