PURPOSE: To obtain the title polymer solution decreased in microgel formed and improved in moldability, by adding a radical polymerization inhibitor to a specified acrylonitrile polymer solution after conducting a derived polymerization.

CONSTITUTION: Acrylonitrile and 0W10mol% vinyl monomer copolymerizable therewith (e.g., acrylic acid) are subjected to a solution polymerization at 40W90°C in an inorganic or organic solvent in the presence of a radical polymerization initiator (e.g., azobisisobutyronitrile) and a chain transfer agent (e.g., dodecyl mercaptan) to obtain an acrylonitrile polymer solution (A) for the production of an acrylonitrile molding. Component A is mixed with at least one radical polymerization inhibitor selected from among (B) an N-nitrosophenyl compound, a hydrazine compound, an aldoxime compound and a mixture of an N-nitrosophenyl compound with a phenyl compound in an amount of at least 1/2mol per mol of the radical polymerization initiator remaining in the acrylonitrile polymer.