A source-drain voltage of one of two transistors connected in series becomes quite small in a set operation (write signal), thus the set operation is performed to the other transistor. In an output operation, two transistors operate as a multi-gate transistor, therefore, a current value can be small in the output operation. In other words, a current can be large in the set operation. Therefore, the set operation can be performed rapidly without being easily influenced by an intersection capacitance and a wiring resistance which are parasitic on a wiring and the like. Further, an influence of variations between adjacent ones can be small as one same transistor is used in the set operation and the output operation.