Temperature-dependent infrared reflectivity spectra of Sr Fe4 Sb12 has been measured. A renormalized Drude peak with a heavy effective mass and a pronounced pseudogap of 10 meV develops in the optical conductivity spectra at low temperatures. As the temperature decreases below 100 K, the effective mass (m*) rapidly increases, and the scattering rate (1 τ) is quenched. The temperature dependence of m* and 1 τ indicates that the hybridization between the Fe 3d spins and the charge carriers plays an important role in determining the physical properties of Sr Fe4 Sb12 at low temperatures. This result is the clear evidence of the iron-based heavy quasiparticles.