The present invention provides a lubricant composition suitable for reducing friction between sliding members such as those within a pump for circulating a heat medium of a heat pump device, and a lubricating system using the composition as a heat medium. A lubricant composition 10 of the present invention comprises: an antifreeze liquid 11 containing ethylene glycol; and 0.01 mass % or less of nanodiamond particles 12. The nanodiamond particles 12 are preferably detonation nanodiamond particles. The lubricant composition 10 is preferably a liquid composition for a heat pump. In addition, the lubricating system of the present invention uses the lubricant composition 10 as a heat medium.