  The magnetic field dependent critical current density $j_c(B)$ of a MgB$_2$
bulk sample has been obtained by means of magnetization hysteresis
measurements. The $j_c(B)$ curves at different temperatures demonstrate a
crossover from single vortex pinning to small-bundle vortex pinning, when the
field is larger than the crossover field $B_{sb}$. The temperature dependence
of the crossover field $B_{sb}(T)$ is in agreement with a model of randomly
distributed weak pinning centers via the spatial fluctuations of the transition
temperature ($\delta T_c$-pinning), while pinning due to the mean free path
fluctuations ($\delta l$-pinning) is not observed.
