The flow pattern in a rotary flow dust collector was changed on a large scale by the flow rates of the primary flow Q₁ and the secondary flow Q₂. Considering the flow characteristics of a turbulent rotational flow, one of the authors newly derived theoretical formulae for the cut-sizes which were determined by another conception in comparison with the derivative means of the cut-size of the ordinary types of cyclones. In these results, the authors confirmed that the relationship between the experimentally determined cut-size Xc₅₀ and the theoretically calculated cut-size Xc showed good coincidence in the range of flow rates Q₂/Q₁ = 1.0∼1.5.