  We have investigated the antiferroquadrupolar(AFQ) order in DyB$_2$C$_2$ by
resonant x-ray scattering. X-rays with energies near the $L_3$ absorption edge
of Dy were employed. Superlattice peaks that correspond to three kinds of
propagation vectors of (1 0 0), (1 0 1/2) and (0 0 1/2) were investigated in
detail with polarization analyses. The experimental results are analyzed using
a formalism on resonant x-ray scattering and a model of the AFQ order. The
magnetic and quadrupolar scatterings are explained by the model satisfactorily.
A detailed investigation on the critical behavior of the AFQ ordering is also
reported. The critical exponent $\beta$ is deduced to be 0.35, not far from the
three-dimensional Heisenberg system. We have also succeeded in detecting the
diffuse scattering above $T_{Q}$.
