PURPOSE: To produce a film excellent in transparency, impact resistance and appearance, by extruding a specified propylene copolymer at a shear rate within a predetermined range.

CONSTITUTION: A propylene copolymer having a melt flow rate of 12g/10min or lower, a density of 0.902g/cm3 or lower and a Barus effect of 1.30 or less is melted by heating, and is extruded through an annular die having an appropriate gap at a shear rate γ satisfying the condition represented by the expression, [wherein γ=6(Q)/(wt2), Q is extrusion volume (cm3/sec), W is the circumference (cm) of the annular die, t is the gap (cm) of the die, MFR is the melt flow rate (g/10min) of the copolymer, and ME is the Barus effect of the copolymer.] A tubular film thus obtained is inflated to a predetermined extent by applying an air or N2 gas pressure to the inside of the film, then it is cooled by blowing air and is flattened.