A branched hetero type monodispersed polyethylene glycol represented by formula (1). (In the formula, each of X1 and Y1 represents an atomic group which contains at least a functional group that forms a covalent bond by reacting with a functional group present in a biologically functional molecule, and the functional group contained in the atomic group X1 and the functional group contained in the atomic group Y1 are different from each other； n is an integer of 6-30 which represents the number of repeating units of the monodispersed polyethylene glycol； E represents a branching moiety which has a divalent bonding valance with respect to L2, while having a monovalent bonding valance with respect to L3, and which is a glycerol moiety； each of L1 and L2 independently represents a single bond or a divalent organic group； L3 represents a single bond, -L4-(CH2)m1- or -L4-(CH2)m2-L5-(CH2)m3-； L4 represents an ether bond, an amide bond or a urethane bond； L5 represents an amide bond or a urethane bond； and each of m1, m2 and m3 independently represents an integer of 1-5.)
