  The temperature dependence of the optical conductivity spectrum of an
itinerant ferromagnetic material, ZrZn$_2$, was obtained and the dynamical
effective mass and the scattering rate were derived. A renormalized Drude peak
with a heavy effective mass developed at low temperatures. The effective mass
rapidly increased below $\hbar\omega$ = 10 meV and at 10 K it was observed to
be 3 times heavier than that at 300 K. The scattering rate also rapidly
decreased below 10 meV at 10 K. These results indicate the creation of heavy
quasiparticles at low temperatures due to spin fluctuations.
