A branched hetero polyethylene glycol according to the present invention is represented by the formula [1]: wherein X and Y represent each an atomic group containing at least a functional group which reacts with a functional group present in a bio-functional molecule to form a covalent bond and the functional group contained in the atomic group X and the functional group contained in the atomic group Y are different from each other; s is an integer of 2 to 8, which represents the number of polyethylene glycol chains; n is the number of average added moles for the polyethylene glycol chain and 20≤n≤2000; and E is a branching linker moiety having s-valent bonding valency for the polyethylene glycol chains and having monovalent bonding valency for the functional group Y.