PURPOSE: To obtain precursor yarn for carbon fiber providing improved properties for separated single filament yarn after being burned, by bringing acrylic yarn after spinning and drawing into contact with a great quantity of water in counter current flow with making it run along the outer peripheral face of a basket roller having a large number of bored through-holes.

CONSTITUTION: In a water washing process to prepare precursor yarn for carbon fiber by spinning a solution of acrylic polymer, drawing it, washing it with water, drying it and making it dense, as a basket roller equipped with through- holes having 5W10mm diameter on the surface of the roller, having 30W50% porosity, is positively rotated in a water bath, and the spun and drawn yarn is wound round the outer peripheral face of the roller in a widen state of ≤5,000 denier/cm yarn density, made to run 15W150m/min, water is made to flow in counter current flow in a mass velocity of 8W20 times the travel speed of the yarn so that the yarn is washed. The washing water has preferably ≤5μmho/cm electric conductivity.