To provide a lubricant high in thermal stability, high in low-temperature fluidity, low in high-temperature volatility, and excellent in lubricating performance and abrasion-proof performance, high in boundary lubricating performance, and also causing no corrosion at high temperatures and high in additive-dissolving ability.

The lubricant consists mainly of one or more ionic liquids that has(have) both cation and anion. In this ionic liquid(s), the cation is at least one kind selected from the group consisting of [R-NC5H5]+, [R-NC4H8]+, [R,R'-N2C3H3]+, [NRxH4-x]+ and [PRxH4-x]+, while the anion is at least one kind selected from the group consisting of BF4-, PF6-, CF3SO3-, (CH3SO2)3C- and (CH3SO2)2N-, R and R' in the cation are each H or a 1-16C alkyl, X in the cation is an integer of 1-3, and especially the cation is a kind of [R,R'-N2C3H3]+ with a phosphoric ester as a functional group.