To provide a waterproof structure for a roof floor face or the like and a waterproofing work undertaking method bringing about high waterproofing and heat insulating effect while saving labor for the work by adopting a simple unit construction method in a combined construction method combining an FRP material and a heat insulating material.

A waterproof plate 4 formed in thin plate shape by molding a composite material (FRP) with glass fiber mixed in polyester resin is stuck to the surface of a heat insulating material 2 slightly larger than the waterproof plate 4 and formed of foam resin of rectangular shape with a specified thickness, and a bonding margin 5 is provided all around the waterproof plate 4 to form a waterproofing unit 1. A plurality of waterproofing units 1 are laid in parallel and stuck onto the backing surface of the roof floor face or the like. After sticking protective tapes 8 to the bonding margins 5, 5 on the adjacent surfaces of the respective waterproofing units 1, a waterproof resin liquid 6 is applied to waterproof the backing surface.