An organodisulfide with a pair of adamantane moieties was synthesized, and its self-assembled monolayer (SAM) was formed on Au(111). The adamantane moieties are almost spherical and much bulkier than alkyl chains. The structure was characterized by scanning tunneling microscopy. Two-dimensional crystals of the SAM were found to be four orientationally different hexagonals with almost the same lattice constant with 4root3a/3 and 7a/3 (a = 0.2884 nm, the Au lattice constant). The structure is assigned to four of the high-order commensurate adlayers. The present study of geometry and energetics for self-assembling of such an organosulfur compound with spherical cages provides a new insight into the probable SAM structure of various thiolate derivatives on Au(111).