The novel intermediate compound which can be efficiently utilized in the synthesis of carotenoid compounds based on the sulfone chemistry, the preparation method of the same, and the practical synthetic process for preparing .beta.-carotene by the use of the above novel compound are disclosed. The synthesis of .beta.-carotene is characterized by the double elimination reactions of the C.sub.40 compound containing both the benzenesulfonyl group and the group X (either halogen or ether), which can be prepared by the coupling reaction of the novel C.sub.10 dialdehyde with two equivalents of the C.sub.15 allylic sulfone, followed by the functional group transformation of the resulting C.sub.40 diol either to the corresponding halide or to the ether, to produce the fully conjugated polyene chain.