  We report the magnetic-field dependence of the irreversible magnetization of
the recently discovered binary superconductor MgB$_{2}$. For the temperature
region of $T< 0.9T_c$, the contribution of the bulk pinning to the
magnetization overwhelms that of the surface pinning. This was evident from the
fact that the magnetization curves, $M(H)$, were well described by the
critical-state model without considering the surface pinning effect. It was
also found that the $M(H)$ curves at various temperatures scaled when the field
and the magnetization were normalized by the characteristic scaling factors
$H^\ast(T)$ and $M^\ast(T)$, respectively. This feature suggests that the
pinning mechanism determining the hysteresis in $M(H)$ is unique below $T=T_c$.
