  The cluster structures of the excited states in $^{11}$B are studied by
analyzing the isoscalar monopole and quadrupole strengths in the
$^{11}$B($d$,$d'$) reaction at $E_d=200$ MeV. The excitation strengths are
compared with the predictions by the shell-model and antisymmetrized
molecular-dynamics (AMD) calculations. It is found that the large monopole
strength for the $3/2^-_3$ state at $E_x=8.56$ MeV is well described by the AMD
calculation and is an evidence for a developed $2\alpha+t$ cluster structure.
