In a conventional analog buffer circuit composed of polycrystalline semiconductor TFTs, a variation in the output is large. Thus, a measure such as to provide a correction circuit has been taken. However, there has been such a problem that a circuit and driver operation are complicated. Therefore, a gate length and a gate width of a TFT composing an analog buffer circuit is set to be larger. Also, a multi-gate structure is adopted thereto. In addition, the arrangement of channel regions is devised. Thus, the analog buffer circuit having a small variation is obtained without using a correction circuit, and a semiconductor device having a small variation can be provided.