PURPOSE: To provide a biodegradable foamed sheet which is light-weight and superior in mechanical characteristics by a method wherein the cap and back film are made of a biodegradable thermoplastic resin, and the film thickness at the thinnest part of an air room is made a specified value or less, and the film thickness ratio of the thinnest part and thickest part of the air room is set at a specified value range, and in addition, the thickness ratio of the cap and back film is set at a specified value range.

CONSTITUTION: A foamed sheet 1 is formed by pasting and integrating a cap film with a large number of protruding parts on a flat back film 3, and a large number of independent air rooms 4 are formed. For the raw material of the cap film and back film, a biodegradable aliphatic polyester resin is used. The film thicknesses of the cap film and back film 3 are not uniform, and the thinnest part exists in a top point 5 of the air room 4. A film thickness 5t at the thinnest part of the air room is made 20μm or less, and the ratio with a film thickness 6t at the thickest part of the air room, i.e., 5t:6t is set at a range of (5t:6t)=(1:2)-(2:3). and the ratio of the cap film and back film 3 is set at a range of (2t:3t)=(2:1)-(1:1).