  Magnetic relaxation of high-pressure synthesized MgB$_2$ bulks with different
thickness is investigated. It is found that the superconducting dia-magnetic
moment depends on time in a logarithmic way; the flux-creep activation energy
decreases linearly with the current density (as expected by Kim-Anderson
model); and the activation energy increases linearly with the thickness of
sample when it is thinner than about 1 mm. These features suggest that the
vortices in the MgB$_2$ are rather rigid, and the pinning and creep can be well
described by Kim-Anderson model.
