There provided a lubricant composition that without any excess increase of the steering torque of an electric power steering apparatus and without any generation of sliding noise, reduces rattling sound thereby excel in the effect of noise reduction in automobile compartment, and is capable of favorably sustaining the above effect even when continuously used in high-temperature environment for a prolonged period of time; a speed reduction gear using the lubricant composition; and an electric power steering apparatus. The lubricant composition includes buffer particles of a polyurethane resin synthesized by reacting a polycarbonatepolyol, cross-linking agent having three or more active hydrogen groups per molecule and a polyisocyanate. The speed reduction gear (50) is filled with the lubricant composition. The electric power steering apparatus incorporates the speed reduction gear therein.