Atrial fibrillation or flutter detector having impedance unit, with measurement input, connected to atrial electrode line having electrode for unipolar measurement of atrium impedance and implemented to generate atrial impedance signal obtained in unipolar manner so that impedance signal comprises multiple impedance values detected at different instants within particular atrial cycle for each atrial cycle, comprising atrial contraction and the following relaxation of the atrium, and having a signal input, via which ventricle signal is supplied to detector, which reflects instants of ventricular contractions in chronological assignment to impedance signal, the detector having an analysis unit, implemented to average multiple sequential impedance signal sections of unipolar atrial impedance signal, delimited by two sequential ventricular contractions with one another and determine maximum amplitude of averaged unipolar atrial impedance signal section, compare to comparison value, and if maximum amplitude of averaged unipolar atrial impedance signal