Adverse effects of variation in threshold voltage are reduced. In a semiconductor device, electric charge is accumulated in a capacitor provided between a gate and a source of a transistor, and then, the electric charge accumulated in the capacitor is discharged; thus, the threshold voltage of the transistor is obtained. After that, current flows to a load. In the semiconductor device, the potential of one terminal of the capacitor is set higher than the potential of a source line, and the potential of the source line is set lower than the potential of a power supply line and the cathode side potential of the load.