We report the magnetic-field dependence of the irreversible magnetization of 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 reversible magnetization and 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 caling 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 $T;=;T_c$.