To provide a fibrillation detector and a defibrillator that are capable of correctly detecting within a short time a ventricular fibrillation (VF) that shows an irregular amplitude or shape, an R-wave detection unit R_DETECT_MEAN converts power of a frequency component making up an R wave of an electrocardio signal ECG into an approximately DC component, and outputs the converted power as R-wave power V_R. A T-wave detection unit T_DETECT_MEAN converts power of a frequency component making up a T wave of the electrocardio signal ECG into an approximately DC component, and outputs the converted power as T-wave power V_T. The R-wave power V_R and the T-wave power V_T are input to a comparison unit CMP, and the comparison unit CMP outputs a magnitude comparison result between the R-wave power V_R and the T-wave power V_T as a comparison signal sig_comp.