It is an object of the present invention to obtain a transistor with a high ON current including a silicide layer without increasing the number of steps. A semiconductor device comprising the transistor includes a first region in which a thickness is increased from an edge on a channel formation region side and a second region in which a thickness is more uniform than that of the first region. The first and second region are separated by a line which is perpendicular to a horizontal line and passes through a point where a line, which passes through the edge of the silicide layer and forms an angle θ (0°