In a method of applying a voltage pulse for injecting/extracting electrons into/from a non-volatile semiconductor memory in which high and low levels of a threshold voltage corresponding to presence and absence of storage of electrons are caused to correspond to binary information, the method includes the steps of generating a plurality of voltage pulses each having an ability of injecting or extracting only a portion of all electrons to be stored, and applying the plurality of voltage pulses to the non-volatile semiconductor memory to thereby carry out injection/extraction of all the electrons.