The application concerns a device for preventing animals coming up a drain pipe. The device can be inserted into a drain pipe (10, 12) of a toilet connected to the discharge network (12) for waste water, and comprises a mobile divider (11) with with a hinge (13) at the top. The divider is able to swing in the direction of the outflow under the pressure of the flow of sewage, freeing the section for the outflow. The divider returns under the action of its own weight to a practically complete closed rest position thus preventing animals (15) coming through the discharge pipes (12).