The present invention provides branched polyalkylene glycols useful as a chemically modifying agent for physiologically active polypeptides, wherein two single-chain polyalkylene glycols are linked to a group having a cyclic structure other than a plane structure, and wherein a group having reactivity with an amino acid side chain, an N-terminal amino group or a C-terminal carboxyl group in a polypeptide or a group convertible into the group having reactivity is linked to the group having a structure other than a plane structure.