We give a construction of the genus field for Kummer ℓnℓn-cyclic extensions of rational congruence function fields, where ℓℓ is a prime number. First, we compute the genus field of a field contained in a cyclotomic function field, and then for the general case. This generalizes the result obtained by Peng for a Kummer ℓℓ-cyclic extension. Finally, we study the extension (K1K2)𝔤𝔢/(K1)𝔤𝔢(K2)𝔤𝔢, for K1, K2 abelian extensions of k.