Abstract: To measure surface potentials in a liquid, the in-liquid potential measurement device according to the present invention includes: a cantilever having a probe at its free end; a displacement measurement unit that measures a voltage corresponding to a displacement of a tip of the cantilever; an AC source that applies an AC voltage between the probe and the sample; and a signal detection unit. A frequency of the AC voltage is 10 kHz or higher. The signal detection unit detects, from the voltage measured by the displacement measurement unit, an amplitude of a frequency component having the same frequency as that of the AC voltage, an amplitude of a frequency component having double frequency of that of the AC voltage, and a frequency component having the same phase as that of the frequency of the AC voltage.