The dynamic behaviour of surface dimers on Ge(001) has been investigated using a variable-temperature (VT) STM. It is clarified that symmetric-appearing dimers, which are generally observed in terraces of Ge(001) at room temperature by STM, are inherently buckled and that the flip-flop motion of buckled dimers is the origin of their symmetric appearance. The evidence of the flip-flop motion was detected by monitoring tunnelling current at positions above selected atoms in symmetric-appearing dimers, as a function of time. It is found that event rate of the flip-flop motion taking place in symmetric-appearing dimers is lowered considerably with decreasing temperature. It is demonstrated that the time-dependent tunnelling-current measurement at positions above selected surface-atoms by using atom-resolved VT-STM provides fruitful information concerning dynamic processes of surface atoms.