The present invention relates to a method of producing a biodegradable resin composition, which includes step (1) of mixing a biodegradable resin, a plasticizer, and a crystal nucleating agent with one another at the melting point (Tm) of the biodegradable resin or more, wherein the crystal nucleating agent is an aliphatic compound having, in its molecule, two or more of at least one group selected from an ester group, a hydroxyl group and an amide group, and step (2) of thermally treating the resulting biodegradable resin composition at a temperature of from the glass transition temperature (Tg) to less than Tm of the composition, as well as a biodegradable resin composition, which contains a biodegradable resin, a plasticizer, and the above crystal nucleating agent, and which satisfies the following conditions: the haze thereof with a thickness of 0.5 mm after thermal treatment at 60.degree. C. for 36 or 60 hours is 20% or less; the storage elastic modulus (E') at a temperature of 25.degree. C. and a frequency of 50 Hz is 1.times.10.sup.8 to 2.times.10.sup.9 Pa; and the storage elastic modulus (E') at a temperature of 60.degree. C. and a frequency of 50 Hz is 1.times.10.sup.7 to 1.times.10.sup.9 Pa.