Precursor of fatal arrhythmia is detected and by setting the tolerance level for carrying out nerve stimulation depending on the severity degree of the precursor, significant variation of the heart rate is suppressed and induction of the fatal arrhythmia is repressed. For this purpose, precursor of tachyarrhythmia occurrence such as premature contraction, ST change, repolarization abnormality or the like is detected and the lower-limit of the atrium interval which becomes the tolerance level of the nerve stimulation is set according to the detected result. Then, it is constituted according to the precursor of aforesaid tachyarrhythmia such that time for making the nerve stimulation means in an operation state is to be adjusted or nerve stimulation is to be carried out as much as the heart rate of a predetermined number of times.