The behavior of a floe in a flow field is analyzed theoretically; in particular, the force it experienced is estimated. Here, a floc is simulated by an entity having a two-layer type of structure, and its porous nature mimicked by varying the relative magnitudes of the permeability of its inner layer and that of its outer layer. The results of numerical simulation reveal that, for the same volume-averaged permeability, the drag coefficient of a floe with a heterogeneous structure is always much larger than that of a floe with a homogeneous structure. This is true regardless of the relative magnitudes of the permeability of the inner layer and that of the outer layer. The drag coefficient of a floe is mainly determined by the part having a less porous structure. We show that for the same volume-averaged permeability, the more heterogeneous the structure of a floe is, the easier for the relation between the drag coefficient and the Reynolds number to deviate from a Stokes' law-like correlation.