Experimental observation has shown that a slightly tilted sharp interface between two convection layers in double-diffusive natural convection migrates perpetually upward gradually with time. This movement of an interface cannot be explained by a simple mathematical model of constant physical properties. The present paper studies the numerical analyses of two-layer convection with the temperature dependence of the properties of the fluid. The perpetual upward migration of an interface was found to be promoted mainly by the temperature dependence of the volumetric coefficient of thermal expansion and also by that of the kinematic viscosity. However, the diffusion coefficient was independent of the migration. The upward migration of an interface appears to be caused by the difference between the intensity of etching due to the flow along the hot wall in the lower layer and that along the cold wall in the upper layer.