The object of this invention is to provide a method for purifying PHA separated from PHA-containing microbial cells in high purity without incurring any serious decrease in molecular weight. The present invention provides a method for purifying a 3-hydroxyalkanoic acid copolymer produced by a microorganism, which comprises treating an aqueous suspension containing the 3-hydroxyalkanoic acid copolymer separated from a microorganism with a hydrogen peroxide while controlling the pH of said aqueous suspension by adding an alkali either continuously or intermittently.