A lubricating structure for a speed reducer includes: a case; a reduction gear provided inside the case, the reduction gear being configured to rotate around a rotation axis; a catch tank configured to store part of lubricating oil scooped up by the reduction gear; an oil passage provided in the side wall, the oil passage being configured to guide the lubricating oil, scooped up by the reduction gear, to the catch tank; a rib formed on the side wall so as to radially extend, the rib being configured to guide the scooped-up lubricating oil toward a radially inner side; and a groove provided at an end edge of the rib of the side wall, the lubricating oil trapped by the rib being introduced into the groove, and the groove communicating with the lubricating oil supply object.