a degradable polymer network obtained by adding from 0 . 2 to 10 parts by weight of a foaming agent to 100 parts by weight of a polylactic acid - base resin composition comprising from 80 to 100 % by weight of polylactic acid or a lactic acid - hydroxycarboxylic acid copolymer and from 0 to 20 % by weight of a plasticizer , conducting melt - foaming extrusion of resulting mixture , and opening the foam cells in the extrudate . the high polymer network is characterized in suitable flexibility and hydrolyzability and thus useful as a material for absorbing oils and body fluids , screens for sunlight , heat - insulating materials , filter media and packaging materials ; and does not accumulate as industrial wastes because of hydrolyzability in the natural environment , even though abandoned after use .