Method for producing biodegradable resin expanded sheet

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

• 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;
• sliding-contacting an external peripheral surface of a cooling mandrel having a diameter larger than that of said annular 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
• cutting the cylindrical expanded sheet that has been subjected to diameter enlargement 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 annular discharge port, and before being in sliding contact with the external peripheral surface of the cooling mandrel, becomes 50° C. or higher and 70° C. or lower.