Disclosed are a lubricant composition including a lubricating base oil, fine particles and a calcium sulfonate-based thickener, a speed reduction gear filled with the lubricant composition, and an electric power steering apparatus including the speed reduction gear. The invention reduces operating noise of the speed reduction gear, regardless of the amount of backlash that occurs on combining a small gear and a large gear and without a more complicated structure for the speed reduction gear, by action of the fine particles. Further, separation of oil is prevented, by action of the calcium sulfonate-based thickener of the lubricant composition, so that electric power steering apparatus noise within the inside of a car is reduced at a low cost.