  The temperature ($T$) dependence of the optical conductivity spectra
$\sigma(\omega)$ of a single crystal SrRuO$_3$ thin film is studied over a $T$
range from 5 to 450 K. We observed significant $T$ dependence of the spectral
weights of the charge transfer and interband $d$-$d$ transitions across the
ferromagnetic Curie temperature ($T_c$ ~ 150 K). Such $T$ dependence was
attributed to the increase in the Ru spin moment, which is consistent with the
results of density functional theory calculations. $T$ scans of $\sigma(\Omega,
T)$ at fixed frequencies $\Omega$ reveal a clear $T^2$ dependence below $T_c$,
demonstrating that the Stoner mechanism is involved in the evolution of the
electronic structure. In addition, $\sigma(\Omega, T)$ continues to evolve at
temperatures above $T_c$, indicating that the local spin moment persists in the
paramagnetic state. This suggests that SrRuO$_3$ is an intriguing oxide system
with itinerant ferromagnetism.
