  We reported far-infrared transmission measurements on a c-axis oriented
superconducting MgB$_{2}$ thin film in the frequency range of 30 $\sim$ 250
cm$^{-1}$. We found that these measurements were sensitive to values of
scattering rate $1/\tau$ and superconducting gap $2\Delta$. By fitting the
experimental transmission spectra at 40 K and below, we obtained $1/\tau =$
(700 $\sim$ 1000) cm$^{-1}$ and $2\Delta (0)\cong$ 42 cm$^{-1}$. These two
quantities suggested that MgB$_{2}$ belong to the dirty limit.
