A mixer in a solution casting system mixes liquid additive with polymer dope fed through a flow line and constituted by cellulose ester as polymer and solvent. A mixer housing is in the flow line for passing the polymer dope through to be mixed. A supply conduit causes flow of the liquid additive for addition to the polymer dope. A distribution channel is formed with the supply conduit, positioned in the flow line, for spreading the liquid additive in a transverse direction of a flow passage in the flow line, to eject the liquid additive. A rotor hub is shaped with a decreasing diameter, contained in the mixer housing, opposed to the distribution channel, and directs the polymer dope in a ring shape after addition of the liquid additive. The flow line collects the polymer dope in a position downstream from the rotor hub.