An electric potential difference detection method detects an electric potential difference between a surface of a sample and a probe of a cantilever in a scanning probe microscope. An AC voltage having a frequency that is ½ of a resonance frequency of the cantilever is applied between the sample and the cantilever, and a magnitude of an amplitude of vibration of the cantilever is detected. On the basis of the detection, a determination is made as to whether an electric potential difference exists or does not exist between the surface of the sample and the cantilever probe. A determination that an electric potential difference between the surface of the sample and the cantilever probe does not exist is made in a case where the cantilever is resonating and the detected magnitude of the amplitude of vibration of the cantilever is greater than a predetermined magnitude.