In order to increase the speed, reduce the number of components and lower power consumption in a sense amplifier circuit for reading data held in a selected memory cell, current difference appearing on a data line is converted to potential difference directly by a current mirror circuit in accordance with data held in a selected memory cell. This current mirror circuit includes a diode for clamping the potential of the data line, and a transistor which is connected with this diode in a current mirror arrangement. A sense current flows through the diode in accordance with the data held in the selected memory cell, a corresponding current flows through the transistor by a current mirror operation, and the sense current is monitored. The monitored sense current is converted to a voltage signal indicating data of the selected memory cell.