A heterobifunctional monodispersed polyethylene glycol represented by formula (1) which has two adjacent monodispersed polyethylene glycol side chains and does not have a chiral center in the molecular structure:wherein X1 and Y1 are each an atomic group containing a functional group capable of forming a covalent bond upon a reaction with a functional group present in a biofunctional molecule. R1 is a hydrocarbon group or the like. n is an integer of 3 to 72. A1 represents -L1-(CH2)m1—； or the like, L1 and L2 represent each an ether bond or the like, and m1 and m2 each independently represent an integer of 1 to 5. B1 represents -L3-(CH2)m3—； or the like, L3 and L4 represent an amide bond or the like, and m3 and m4 each independently represent an integer of 1 to 5. Also disclosed is an antibody-drug conjugate including the heterobifunctional monodispersed polyethylene glycol.
