  We investigate the low-lying 2$^+$ states of $N=28$ isotones ($^{48}$Ca,
$^{46}$Ar, $^{44}$S and $^{42}$Si) with using the canonical-basis
time-dependent Hartree-Fock-Bogoliubov theory in which the pairing is taken
into account self-consistently. The quadrupole mode with very small excitation
energies emerges in $^{46}$Ar, $^{44}$S and $^{42}$Si due to the strong
quadrupole correlations triggered by the quenching of the $N=28$ shell gap. The
importance of the quadrupole correlations between the protons and neutrons, and
the role of the pairing correlation to reinforce the lowest 2$^+$ mode are
discussed. It is also shown that the $B$(E2$\uparrow$) values of $^{48}$Ca and
$^{46}$Ar are plausibly explained by our calculations.
