next the width of the depletion layer 12 is set again to the predetermined value w1 the potential difference between the first control gate electrode 7 and the second control gate electrode 9 is set again to the predetermined value e1 to implant hot electrons to the portion 5a of the floating gate electrode 5 thus as shown in fig9 b the amount of hot electrons implanted to the portion 5a of the floating gate electrode 5 is substantially twice that where the data value is 34 00 34 in other words the amount of hot electrons implanted to the floating gate electrode is increased in the vertical direction in this case the electrons are accumulated in the portion 5a only but not in the portion 5c excluding the portion 5a the width of the depletion layer 12 is set to the predetermined value w1 as shown in fig8 a the potential difference between the first control gate electrode 7 and the second control gate electrode 9 is set to the predetermined value e1 to implant hot electrons into the portion 5a of the floating gate electrode 5 next the width of the depletion layer 12 is set to the predetermined value w2 as shown in fig8 b the potential difference between the first control gate electrode 7 and the second control gate electrode 9 is set to the predetermined value e1 to implant hot electrons to the portion 5b of the floating.