We deduced the ζ potential of various rocks in Aso caldera from streaming potential measurements in laboratory. In contrast to the conventional observation that the ζ potential of crustal rocks is mostly negative under typical geologic conditions, almost half of the samples equilibrated with dilute KCl solution show positive values of ζ potential at pH 4.5--5.5. The samples showing positive ζ are characterized by relatively low SiO2 content and abundance of elements having high isoelectric points, and found to be localized around the summit of Mt. Takadake where the positive correlation between self-potential (SP) and altitude is observed. We emphasize that measurements of ζ potential of volcanic rocks are very important to give more appropriate interpretations of SP data from volcanic fields.