Abstract: A method of preventing water leakage which comprises the steps of interposing a waterproof seal which consists of a hollow, water-absorbing, swelling rubber piece covered with a fabric tube in a clearance between the ends of pipes or planks to be joined together, injecting air into the hollow of the rubber piece and thereby block off the clearance and obtain a contacting surface pressure greater than a leakproof pressure just needed, and thereafter allowing the water-absorbing, swelling rubber piece to swell with water. The waterproof seal may have an inlet valve through which air is forced into the hollow, or the seal may be pierced through with a hollow needle from the outside to inject air into the hollow.