In a semiconductor device including a multi-gate MIS transistor having a channel on a plurality of surfaces, a gate electrode is formed on a gate insulating film on side surfaces of an island-like semiconductor layer formed along a given direction on an insulating film, and source/drain electrodes are formed in contact with the semiconductor layer. The semiconductor layer has a plurality of side surfaces along the given direction. All angles formed by adjacent side surfaces are larger than 90.degree.. A section perpendicular to the given direction is vertically and horizontally symmetrical.