A gear assembly having first and second gears in constant mesh with each other, the second gear being divided into two gear elements by a plane perpendicular to the axis thereof. A passage through which lubricating oil flows is formed between these two gear elements. The passage preferably consists of a radial groove formed on a surface of one of the two gear elements to extend in a radial direction from the inner surface of the gear element, an annular groove formed on the surface to extend in a circular direction about the axis of the gear element and communicate with the outer end portion of the radial groove, and a clearance formed between the two gear elements and surrounding the annular groove.