In classical number theory, one has the famous theorem of Kummer-Sinnott giving the index of the cyclotomic units in the total unit group. Using the division values of sgn-normalized rank one Drinfeld modules, we construct the group of extended cyclotomic units for an abelian extension of a global function field and calculate its index in the whole unit group. The result could be regarded as an analogue and a generalization of Kummer-Sinnott's.