To provide a waterproof structure of a placing joint, which has excellent waterproof properties, by sealing not only a void around a water swelling member but also a fine void such as an air bubble or a crack inside existing concrete or placing joint concrete and a void separate from the water swelling member.

Water swelling rubber 6 interposed between the placing joints 2 absorbs infiltration inflow and undergoes swelling so as to enable the sealing of a placing joint void 5 around the rubber 6. Furthermore, because the rubber 6 contains permeable silica, the infiltration inflow is infiltrated into the rubber 6 for the purpose of dissolution of the permeable silica contained in the rubber 6. The dissolved permeable silica is leached out of the rubber 6, and the leached permeable silica reaches the void 50, such as the air bubble or the crack inside the concrete, separate from the rubber 6 and undergoes solidification by reaction with the concrete so as to be made into silica gel. Consequently, even the void 5 separate from the rubber 6 can be sealed in order to enable the acquirement of a more reliable waterproof function.