We have developed novel watertight and bioabsorbable synthetic dural substitutes. The substitutes were designed such that they had a three-layered structure, and each layer comprised a bioabsorbable copolymer prepared from L-lactide, glycolide, and epsilon-caprolactone. Various copolymers were synthesized, and appropriate compositions were selected for preparing the substitutes based on the results of leakage tests. Experimental substitutes that prevented the leakage of saline through the suture lines between the substitutes and dura mater were prepared. An in vitro study was performed in which the substitutes were sutured to porcine dura mater, and the amount of saline leakage was measured. It was demonstrated that leakage through the suture line could be completely suppressed by using the substitutes we developed along with favorable suturing techniques.