Lubricant and its use for a magnetic recording medium and a slider

There is provided a lubricant, which achieves a sufficiently small one-molecule layer film thickness, even if it has a large molecular weight, and which has improved reliability in an environment having a wide temperature range without losing the flight stability by making the film thickness of a lubricant layer very small when it is used for the lubricant layer for a magnetic recording medium and / or lubricant layer for a slider of a magnetic recording apparatus. This lubricant contains a fluorine-containing polymer, wherein the lubricant is a relationship of: Y comprises <-1,4475X + 2,815 wherein Y is a natural logarithm of the diffusion coefficient at 23 ° C (unit: microns and X is a viscosity at 20 ° C (unit: Pas), or it contains a fluorine-containing polymer having a particular structure, and with molecular chains between adjacent polar groups, which have a molecular weight number average of not less than 500th