  We discuss damping of the collective rotational motion in $A\sim 150$
superdeformed nuclei by means of a shell model combining the cranked Nilsson
mean-filed and the surface-delta two-body residual force. It is shown that,
because of the shell structure associated with the superdeformed mean-field,
onset energy of the rotational damping becomes $E_x \sim 2-3 $ MeV above yrast
line, which is much higher than in normal deformed nuclei. The mechanism of the
shell structure effect is investigated through detailed analysis of level
densities in superdeformed nuclei. It is predicted the onset of damping varies
in different supedeformed nuclei along with variation in the single-particle
structure at the Fermi surface.
