A multi-gate structure is used and a width (d1) of a high concentration impurity region sandwiched by two channel forming regions in a channel length direction is set to be shorter than a width (d2) of low concentration impurity regions in the channel length direction. Thus, a resistance of the entire semiconductor layer of a TFT which is in an on state is reduced to increase an on current. In addition, a carrier life time due to photoexcitation produced in the high concentration impurity region can be shortened to reduce light sensitivity.