as in the conventional example a source potential of the memory transistor is about 0 1 v or less this value is within the permissible error range u00b1 0 3 v of the high voltage vpp and the writing characteristics are considerably improved compared with the conventional device as a result even if the sheet resistance value is increased to some extent as a result of increase of the storage capacity good writing characteristics can be ensured the same applies to the reading characteristics if the memory transistor is connected to the first bit line bl 2 from the write source line sl r the following conditions are also obtained thus the composite resistance rs 2 between the source region of the memory transistor and the source line sl with respect to the impurity region 11 is entirely the same as rs 1 accordingly there is no difference in the source potential of the memory transistor due to a difference of bit lines bl connected thereto in other words the electric characteristics of the memory transistor are always constant irrespective of the difference in the location where the memory transistor is formed fig1 is an equivalent circuit diagram of a portion including a floating gate and a control gate referring to fig1 a capacitor ca is formed between the control gate 15a to.