To provide a solution film forming method for obtaining a thin film excellent in optical isotropy.

A dope 12 is prepared using a solvent wherein dichloromethane and an alcohole being a poor solvent are mixed. The alcohol 20 is sent to the dope in in-line piping 16 to be mixed with the dope by a static mixer 30 to obtain a casting dope 18 enhanced in the compositional ratio of the alcohol. The temperature of a rotary drum 32 is adjusted to -7°C and the casting dope 18 is cast on a rotary drum 32 from a casting die 31 so that a film thickness becomes 40 μm to form a cast film 33. Since the compositional ratio of the alcohol is high and the storage modulus of the cooled cast film 33 becomes 150,000 Pa or above, a soft film 38 causing no peeling failure at the time of peeling and minimally suppressed in stretching is obtained. The soft film 38 is dried in a tenter type dryer 50 to be stretched so that the elongation thereof becomes 110% or below. A film 51 has a reduced thickness and is excellent in surface properties and optical isotropy.