A branched type hetero monodispersed polyethylene glycol represented by formula (1) (X1 and Y1 each represents an atomic group containing at least a functional group capable of reacting with a functional group present in a biofunctional molecule to form a covalent bond, provided that 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 represents a number of repeating units of a monodispersed polyethylene glycol and is an integer of 6 to 30, E is a branch site having a divalent bond valence number to L2 and a monovalent bond valence number to L3 and represents a glycerol site, L1 and L2 each independently represents a single bond or a divalent organic group, and L3 represents a single bond, -L4-(CH2)m1- or -L4-(CH2)m2-L5-(CH2)m3-, L4 represents any of an ether bond, an amide bond and a urethane bond, L5 represents an amide bond or a urethane bond, and m1, m2 and m3 each independently represents an integer of 1 to 5.)
