An oil-in-water type (O/W) emulsion lubricant composition includes safe and renewable components that do not have environmental load, exhibits excellent lubrication properties, has excellent handling capability, is economical, and may be applied to tribological fields.The lubricant composition includes water or oil dispersant-treated ultrafine diamond particles in the aqueous phase (W phase) and/or the oil phase (O phase) of an oil-in-water (O/W) emulsion, or includes at least one multiple emulsion state that includes an oiliness improver and/or a composite state that includes a solid lubricant other than the ultrafine diamond particles in the same configuration.