This heartbeat detecting device is provided with: a time difference value calculation unit (3) for calculating a time difference value from a sampling data sequence of an electrocardiogram waveform ofa subject; an index value calculation unit (4) for calculating an index value for detection of a heartbeat on the basis of the time difference value; a threshold value setting unit (5) that determines whether peaks of the index value that exceed a threshold value are detected or not, calculates, when a predetermined number or more peaks are continuously detected, a candidate threshold value on the basis of the average of a predetermined number of the latest peaks among the peaks, does not update the threshold value when the candidate threshold value exceeds a threshold value limit based on adifference limit that is likely to be the time difference value of the electrocardiogram waveform of the subject, and sets the candidate threshold value as a new threshold value when the candidate threshold value is not more than the threshold value limit; and a heartbeat time determination unit (6) that determines, when a peak of the index value that exceeds the threshold value is detected, the sampling time of the peak as a heartbeat time.