PURPOSE: To shorten both net polishing time and changing time of a polishing liquid by changing a polishing liquid fed to a polishing device over to the polishing liquid of small polishing particle from that of a large one under operation of a polishing device.

CONSTITUTION: A polishing liquid including the polishing particle which polishes the surface roughness of a work (aluminum substrate for magnetic disk) A in the specified surface roughness, and one kind or plural kinds of polishing liquids including the polishing particle of a relatively large grain size which polishes the surface roughness of a work in the surface roughness larger than that obtained by the use of the polishing liquid are prepared. Then the polishing liquid fed to a polishing device 1 is changed over to the polishing liquid of small polishing particle from a polishing liquid of large polishing particle under the operation of the polishing device 1. The necessary time for polishing the work A in the specified surface roughness can thus be shortened.