A biodegradable disposable diaper comprising a combination of a liquid absorbing material, a liquid permeable surface material and an leakproof backing material, said liquid permeable material being formed of an aliphatic polyester resin obtained by reacting an aliphatic saturated polyester prepolymer having an end group which is materially a hydroxyl group with a coupling agent or formed of said aliphatic polyester resin and an aliphatic saturated polyester resin which has not been treated by said coupling agent. The disposable diaper has excellent mechanical strength, softness, feel of touch, productivity as well as biodegradability, air permeability and waterproofness.