To provide a lubricant composition containing buffer particles capable of reducing rattling sounds still more effectively without increasing excessively the torque of a speed reducer and the steering torque of an electric power steering device while influence of the temperature is excluded as much as practicable, and provide a speed reducer using the same and power steering device.

The lubricant composition contains a lubricant and buffer particles including inorganic or organic hard particulates, wherein the ratio of the mean particle size du of the buffer particles to that ds of the hard particulates is 2,000 or below, and the vol% of the hard particulates per one or more buffer particulates is arranged to be 10 vol% or less when the mean particle size of the buffer particles is 50 m or less. The invention also includes a speed reducer filled with the lubricant composition and an electric power steering device in which the speed reducer is incorporated.