A first semiconductor layer is formed on an insulating surface. A first insulating layer for covering an upper side of the first semiconductor layer is formed. On the first insulating layer, a second semiconductor layer is formed. A second insulating layer for covering an upper side of the second semiconductor layer is formed. A first contact hole extending through the first and second insulating layers to reach the first semiconductor, and a second contact hole extending through the second insulating layer to reach the second semiconductor layer but not reaching the first insulating layer are opened. After the step of forming the second insulating layer before the step of opening the first and second contact holes, laser or heat annealing process is executed.