An implantable cardiac stimulator having an at least partially electrically conductive housing, a ventricular stimulation unit connectable to left ventricular or right ventricular stimulation electrode and designed to generate ventricular stimulation pulses for stimulation of heart ventricle, having terminal for right ventricular defibrillation electrode. Has far-field electrogram detection unit (FFEDU) and stimulation success detecting unit (SSDU), of which FFEDU has first input connected to the terminal for right ventricular defibrillation electrode and second input connected to housing. FFEDU detects far-field electrocardiogram based on electric potentials applied to inputs and deliver these potentials to SSDU. SSDU has electrogram input and signal input receives a far-field electrogram generated by FFEDU and receives stimulation signal that represents a ventricular stimulation pulse generated and delivered by ventricular stimulation unit and analyzes section of received far-field electrogram immediately following delivery of ventricular stimulation impulse to determine whether it represents an effective or ineffective ventricular stimulation.