  We perform three-body model calculations for a $sd$-shell hypernucleus
$^{19}_{\Lambda}$F ($^{17}_{\Lambda}{\rm O}+p+n$) and its core nucleus $^{18}$F
($^{16}{\rm O}+p+n$), employing a density-dependent contact interaction between
the valence proton and neutron. We find that the $B(E2)$ value from the first
excited state (with spin and parity of $I^\pi=3^+$) to the ground state
($I^\pi=1^+$) is slightly decreased by the addition of a $\Lambda$ particle,
which exhibits the so called shrinkage effect of $\Lambda$ particle. We also
show that the excitation energy of the $3^+$ state is reduced in
$^{19}_{\Lambda}$F compared to $^{18}$F, as is observed in a $p$-shell nucleus
$^{6}$Li. We discuss the mechanism of this reduction of the excitation energy,
pointing out that it is caused by a different mechanism from that in
$^{7}_{\Lambda}$Li.
