  The previous mean-field calculation [Myaing Thi Win and K. Hagino, Phys. Rev.
C{\bf 78}, 054311 (2008)] has shown that the oblate deformation in
$^{28,30,32}$Si disappears when a $\Lambda$ particle is added to these nuclei.
We here investigate this phenomenon by taking into account the effects beyond
the mean-field approximation. To this end, we employ the microscopic
particle-rotor model based on the covariant density functional theory. We show
that the deformation of $^{30}$Si does not completely disappear, even though it
is somewhat reduced, after a $\Lambda$ particle is added if the
beyond-mean-field effect is taken into account. We also discuss the impurity
effect of $\Lambda$ particle on the electric quadrupole transition, and show
that an addition of a $\Lambda$ particle leads to a reduction in the $B(E2)$
value, as a consequence of the reduction in the deformation parameter.
