The laser beam projection mask 14 has three rectangular-shaped slits 25,
26, 27 as transmission areas. These three slits 25, 26, 27 are formed in
sequence in X direction shown by an arrow X in FIG. 2C at specified
intervals, and the width in the X direction decreases in the order of the
slit 25, the slit 26 and the slit 27. More particularly, transmission
coefficients of the transmission areas change in conformity with a
temperature distribution curve V1 of a silicon film 4 shown in FIG. 2B.