Time evolution of dust-to-gas mass ratio Σ_grain/Σ_gas at the peak of the dust rings. The black and red curves show the Σ_grain/Σ_gas values at the peak of dust rings in Models 1 and 2: (dotted) the inner ring at ∼5 au, (dashed) the middle ring at ∼11 au, and (solid) the outer ring at ∼32 au. The blue solid curve shows the Σ_grain/Σ_gas value at the peak of the outer dust ring in Model 3 at ∼29 au.