A method detects one or more cracks in a piezoelectric element interposed between a pair of electrodes and deforms according to a voltage applied thereto through the pair of electrodes. The method includes steps of applying a voltage at least at a resonance frequency of the piezoelectric element to the piezoelectric element through the pair of electrodes, measuring a dielectric tangent between the pair of electrodes under the applied voltage, and detecting if there are cracks in the piezoelectric element according to the measured dielectric tangent. The dielectric tangent of the piezoelectric element at the resonance frequency has a large peak or no peak depending on whether or not the piezoelectric element has cracks. Accordingly, the method easily and surely detects if the piezoelectric element has cracks according to the dielectric tangent of the piezoelectric element measured at the resonance frequency.