Time-resolved measurement of photoemission spectroscopy was made to trace a change of surface potential after the surface photovoltage effect on a Si(111) $7\ifmmode\times\else\texttimes\fi{}7$ surface. Two relaxation processes were found with decay times of nanoseconds and hundreds of nanoseconds, which are explained in terms of the tunneling and the thermionic relaxation schemes, respectively. At the high laser power density, the relaxation has become oscillatory with a temporal period of several tens of nanoseconds.