An insulating film 103 for making an under insulating layer 104 is formed on a quartz or semiconductor substrate 100. Recesses 105a to 105d corresponding to recesses 101a to 101d of the substrate 100 are formed on the surface of the insulating film 103. The surface of this insulating film 103 is flattened to form the under insulating layer 104. By this flattening process, the distance L1, L2, . . . , Ln between the recesses 106a, 106b, 106d of the under insulating layer 104 is made 0.3 m or more, and the depth of the respective recesses is made 10 nm or less. The root-mean-square surface roughness of the surface of the under insulating film 104 is made 0.3 nm or less. By this, in the recesses 106a, 106b, 106d, it can be avoided to block crystal growth of the semiconductor thin film, and crystal grain boundaries can be substantially disappeared.