Normal memory cells and dummy cells are arranged continuously in a memory array. In a data read operation, first and second data lines are connected to the selected memory cell and the dummy cell, respectively, and are supplied with operation currents of a differential amplifier. An offset corresponding to a voltage difference between first and second offset control voltages applied from voltage generating circuits are provided between passing currents of the first and second data lines, and a reference current passing through the dummy cell is set to a level intermediate between two kinds of levels corresponding to storage data of a data read current passing through the selected memory cell.