PURPOSE: To eliminate the fluctuation of an access time by changing the characteristic of a sense amplifier at every memory cell string obtained by dividing memory cell array at every string and making the read margin of stored contents '1' of a memory larger in the sense amplifier in the memory cell string as it is farther from a decoder.

CONSTITUTION: By changing the size of a transistor, the read margin of '1' is set larger in the sense amplifier in the memory cell string D0 as it is farther from the decoder. For example, even if the applied current of a selection mem ory in the memory cell string D0 is minute compared with that of the memory cell string D3 as shown in figure (a), its access time becomes the same level as that of the sense amplifier in the memory cell string D3 since the read mar gin of '1' is made to be increased in order to be able to decide the stored contents as '1'. As a result, the respective sense amplifiers read out the stored information '1' from the respective memory cell strings with nearly same access time as shown in figure (c), so that the data output 5 is outputted in nearly same access time without generating the fluctuation among the memory cell strings.