  The explicit form of the interface equation of motion derived assuming a
minimal surface is extended to general bicontinuous interfaces that appear in
the diffusion limited stage of the phase separation process of binary mixtures.
The derivation is based on a formal solution of the equivalent simple layer for
the Dirichlet problem of the Laplace equation with an arbitrary boundary
surface. It is shown that the assumption of a minimal surface used in the
previous linear theory is not necessary, but its bicontinuous nature is the
essential condition required for us to rederive the explicit form of the simple
layer. The de- rived curvature flow equation has a phenomenological cut-off
length, i.e., an `electro-static' screening length. That is re- lated to the
well-known scaling length characterizing the spatial pattern size of a
homogeneously growing bicontinuous phase. The corresponding equation of the
level function in this scheme is given in a one-parameter form also.
