A process for preparing polyphenylene ether by oxidizing phenols using water as a solvent, enabling polyphenylene ether to be prepared with only a small amount of oxidizer, while making it possible to reuse a solvent after reaction repeatedly. Water is used as the solvent. Phenols are oxidized under the presence of a water-soluble metal complex catalyst. For the water-soluble metal complex catalyst, it is preferable to use the one whose central metal is copper or manganese, having an amine multidentate ligand.