Data for detecting magnetic flux surfaces at the edge plasma in tokamaks are presented here. When magnetic flux surfaces are moved by a plasma jog, it is observed that a current increment appears in the two sets of double probes whose front surfaces are shifted by a small distance and that a difference between these two currents through the differential amplifier giving a signal of the arrival of the separatrix also appears; we refer to consider the separatrix point as a typical position of the magnetic flux surfaces. It is confirmed that magnetic:flux surfaces are located a few cm outward from that calculated by the eqfit-code (EQFIT) and that the absolute value of its deviation is identified. This identification is supported by the result of the observation of the correlation of the floating potential fluctuation between two different points using one probe pin of the same differential as the same double probes. The ion temperature and the space potential are observed by the another use of the differential double probe are modified following the estimated real magnetic surfaces positions.