PURPOSE: To perform operations of transistors having different conductance constants by one transistor by forming regions having different widths, lengths and impurity concentrations of channels in the one transistor.

CONSTITUTION: A precharge signal 4p is set to a low level, transistors Qp114, Qp215, QPA16 are interrupted, and a bit line 11 and a node S13 are set to a floating state. Then, word line applying pulses WL, WD of a memory cell and a dummy cell are set to a high level. An information signal and a reference voltage signal are read from the memory cell 17 and the dummy cell 18 to the bit lines 11, 12. A voltage is gradually applied to 0 of a sense amplifier 10, only a part (small conductance constant) of a region 2 of a transistor 1 is operated at the time of low voltages 1a-1b to amplify an infinitesimal signal. A region 3 (large conductance constant) of the transistor 1 is operated at the time of voltages 2b-3a, and abruptly amplified.