The present invention is to provide a polishing technique ensuring that when polishing a to-be-polished surface in the production of a semiconductor integrated circuit device, appropriate polishing rate ratios can be obtained between a borophosphosilicate glass material layer and other materials and high planarization of the to-be-polished surface containing a borophosphosilicate glass material layer can be thereby realized. The present invention relates to a polishing agent for chemical mechanical polishing, containing a cerium oxide particle, a water-soluble polyamine, one or more basic compounds selected from the group consisting of monoethanolamine, ethylethanolamine, diethanolamine and ammonia, and water, wherein the polishing agent has a pH of from 10 to 13 and wherein the basic compound is contained in an amount of more than 0.01 mass %.