The present invention provides a lubricant for forming a lubricating layer which is useful in preventing fly and stiction and/or corrosion at an extremely small flying height of 10 nm or less, which is prevented from migrating at a high rotational speed of 5400 rpm or more, which has high adhesion, and which is suitable for a load/unload method. A crude lubricant containing at least one perfluoropolyether is degassed and then purified. Alternatively, a liquid crude lubricant containing at least one perfluoropolyether is purified by vaporizing the crude lubricant and then liquefying the vapor of the perfluoropolyether within a distance less than the mean free path of molecules of the perfluoropolyether. A magnetic disk is prepared in such a manner that a carbonaceous protective layer is formed above a substrate and a lubricating layer is formed on the protective layer using of the lubricant. The lubricant contains the perfluoropolyether and has a molecular weight distribution of 1.3 or less.