Abstract: The object of the present invention is to provide a drainage basin attachment which can prevent breeding of mosquitoes in a puddle inside the drainage basin and also can shield scattering of an odor from the inside. The attachment includes a supporting member retained to the upper end of the basin body and an inner gate supported by the supporting member beneath the basin lid to close the top opening of the basin body. The inner gate forms an inner basin for temporarily storing the rainwater passing through the basin lid. The inner gate is normally closed and is opened by an action of force acting on the inner gate by the rainwater accumulated in the inner basin to an extent above a predetermined level, thereby dropping the rainwater from the inner basin into the basin body.