The present polishing slurry is a polishing slurry for chemically mechanically polishing a surface of a Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y crystal (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1), characterized in that this polishing slurry contains abrasive grains formed of SiO.sub.2, this abrasive grain is a secondary particle in which a primary particle is associated, and a ratio d.sub.2/d.sub.1 of an average particle diameter d.sub.2 of a secondary particle to an average particle diameter d.sub.1 of a primary particle is not less than 1.6 and not more than 10. According to such the polishing slurry, a crystal surface having a small surface roughness can be formed on a Ga.sub.xIn.sub.1-xAs.sub.yP.sub.1-y crystal at a high polishing rate and effectively.