  We report a comprehensive infrared magneto-spectroscopy study of
CeRu$_4$Sb$_{12}$ compound revealing quasiparticles with heavy effective mass
m$^*$, with a detailed analysis of optical constants in fields up to 17 T. We
find that the applied magnetic field strongly affects the low energy
excitations in the system. In particular, the magnitude of m$^*$ $\simeq$ 70
m$_b$ (m$_b$ is the quasiparticle band mass) at 10 K is suppressed by as much
as 25 % at 17 T. This effect is in quantitative agreement with the mean-field
solution of the periodic Anderson model augmented with a Zeeman term.
