We apply the infinite mass effective theory, when a heavy quark mass t
ends to infinity, and chiral perturbation theory at the quark level, b
ased on the extended Nambu-Jona-Lasinio model with linear realization
of chiral U(3) x U(3) symmetry, to calculate the form factors of the D
-s(+) --> phi mu(+)nu(mu), decay up to the first order in current s-qu
ark mass. The theoretical results are compared with experimental data
and found to be in good agreement.