  Field-dependent magnetic structure of a layered Dirac material EuMnBi$_2$ was
investigated in detail by the single crystal neutron diffraction and the
resonant x-ray magnetic diffraction techniques. On the basis of the reflection
conditions in the antiferromagnetic phase at zero field, the Eu moments were
found to be ordered ferromagnetically within the $ab$ plane and stacked
antiferromagnetically along the $c$ axis in the sequence of up-up-down-down.
Upon the spin-flop transition under the magnetic field parallel to the $c$
axis, the Eu moments are reoriented from the $c$ to the $a$ or $b$ directions
forming two kinds of spin-flop domains, whereas the antiferromagnetic structure
of the Mn sublattice remains intact as revealed by the quantitative analysis of
the change in the neutron diffraction intensities. The present study provides a
concrete basis to discuss the dominant role of the Eu sublattice on the
enhanced two-dimensionality of the Dirac fermion transport in EuMnBi$_2$.
