Provided is a waterproof sheet such that an adhesive layer is formed on a substrate rubber layer and characterized by the substrate rubber layer comprising a silicone rubber having a JIS-A hardness of 15-90, a tensile strength of at least 3 MPa, an elongation of 100-800%, and a crescent tear strength of at least 3 kN/m. The waterproof sheet can be used without a long-term decrease in material properties, and is waterproof in the long-term. In particular, the present invention can be a waterproof sheet able to withstand long-term use by means of stipulating the material properties of the substrate rubber. By means of a method for applying waterproofing to an outdoor tank, it is possible to reliably waterproof the bottom side of the outdoor tank by means of a simple application method.