An object of the present invention is to provide a method for efficiently producing an oligomer or a monomer by degrading a biodegradable resin using an enzyme, so that the oligomer or the monomer can be recovered.The present invention provides a method for producing an oligomer and/or a monomer by degrading a biodegradable resin in a degradation liquid containing a biodegradation enzyme, a buffer agent, an organic solvent, and water. In this method, the SP value of the organic solvent is less than 8.5 or more than 11.5, and the percentage content of the organic solvent (by volume) in the degradation liquid is higher than 1% and lower than 15%. In the method for producing an oligomer or a monomer, the degradation percentage of the biodegradable resin is low, and deposits of aggregates of the oligomer and/or the monomer are few, so that the recovery percentage is high.