The invention provides a semiconductor device which is capable of decreasing an effect of a variation in characteristics of transistors, supplying a predetermined current even when voltage-current characteristics of a load change, and improving a write speed of a signal sufficiently even when the amount of a signal current is small. In the semiconductor device, a current-voltage converting element and a transistor are connected in series; and an amplifier circuit detects a voltage which is applied when a current flows to the current-voltage converting element, and sets a gate-source voltage of the transistor depending on the voltage. Therefore, since the amplifier circuit has low output impedance, a write speed of a signal can be improved sufficiently even when the amount of a signal current is small.