To form a semiconductor thin film having a uniform crystal structure by inducing lateral crystal growth through one laser light irradiation.

A laser light is given to heat and melt the external area 107 of a semiconductor thin film 105 outside the pattern of an optical absorption layer 103, and at the same time, the optical absorption layer 103 is heated without melting the internal area 109 of the semiconductor thin film inside the pattern thereof. Next, the melted semiconductor thin film 105 is cooled, and micro crystal grains S are generated adjacent to a boundary between the external area 107 and the internal area 109. Furthermore, first lateral growth progresses using the micro crystal grains S as nucleus from the boundary toward the outside, and polycrystal grains L1 are generated in the external area 107. Finally, a heat is transmitted to the semiconductor thin film 105 from the heated optical absorption layer 103 and the internal area 109 is melted, and then second lateral growth progresses using the polycrystal grains L1 as nucleus from the boundary toward the inside, and further expanded polycrystal grains L2 are generated in the internal area 109.