Pulse shaped voltage of 5V is applied to a source region 3 at initial phase of erase by a pull back voltage generator 13 connected to the sources region 3. Then, the pulse shaped voltages of 10V and 12V increased under stepwise bases are applied to source region 3 with progress of erasion. Generation of hot-holes at the initial phase of data erasion can be prevented because difference in voltage between the floating gate electrode 5 and source region 3 is decreased. Value of the pulse shaped voltage thus applied is increased for the difference occurred between the floating gate electrode 5 and source region 3 when erasion is in much progress. Thus, it is possible to pull out the stored electrons from the floating gate electrode 5 until the threshold voltages can be set at predetermined values. So that, degradation of characteristics of a gate oxidation layer caused by hot-holes generated with erasion can be prevented.