Method for producing biodegradable resin expanded sheet

A method for producing a biodegradable resin expanded sheet, the method comprising:

• an extrusion expansion step of melting and kneading a resin composition containing a biodegradable resin and a blowing agent with an extruder, and extrusion expanding the melted and kneaded resin composition in a molten state from a circular die having an annular discharge port to form a cylindrical expanded sheet;
• a diameter enlarging step of sliding-contacting an external peripheral surface of a cooling mandrel having a diameter larger than that of said discharge port on an internal surface of said cylindrical expanded sheet to cool said cylindrical expanded sheet from an internal side while enlarging a diameter thereof, and
• a developing step of cutting the cylindrical expanded sheet that has been subjected to diameter enlargement in the diameter enlarging step along an extrusion direction to develop the cylindrical expanded sheet, to obtain a flat belt-like expanded sheet,
• wherein said cylindrical expanded sheet is cooled so that a temperature of an external surface of said cylindrical expanded sheet at a position at which 5 seconds have elapsed after being discharged from said discharge port becomes 50° C. or higher and 70° C. or lower.