To provide a method for producing a polymer, which contains: bringing a ring-opening polymerizable monomer containing an amide bond, and a compressive fluid into contact with each other to melt or dissolve the ring-opening polymerizable monomer containing an amide bond, followed by allowing the ring-opening polymerizable monomer containing an amide bond to react through ring-opening polymerization in the presence of a basic organic metal catalyst and a cocatalyst, to thereby obtain a polymer product.

Provided is a method for preparing a polyamide by an anionic polymerization reaction, wherein lactam, and based on 100 parts by weight of the entire lactam, 0.01 parts by weight to 20 parts by weight of an alkali metal as an initiator, 0.3 parts by weight to 10 parts by weight of a molecular weight controller, and 0.002 parts by weight to 1.0 part by weight of carbon dioxide as an activator may be included.

Provided is a method for preparing a polyamide by an anionic polymerization reaction, wherein lactam, and based on 100 parts by weight of the entire lactam, 0.01 parts by weight to 20 parts by weight of an alkali metal as an initiator, 0.3 parts by weight to 10 parts by weight of a molecular weight controller, and 0.002 parts by weight to 1.0 part by weight of carbon dioxide as an activator may be included.