The methods of configurational average of ensemble on a homogeneously random binary system is extended to treat a conditionally random binary system, where the concentration of one species c(r) is not a constant but modulated in a certain way. A restricted ensemble is chosen to describe such a system, the corresponding generalized CPA equation is derived. If we separate c(r) into c and δc(r), a decomposition scheme is introduced to average the uniform part c and deviation part dc successively. For a sinusoidal modulation, the averaged single-particle Green's function and its self-energy are calculated formally to the second order in δc/c. In the virtual crystal limit, the band splitting character of a superlattice is recovered.