To provide a high-speed combinational circuit including an S-Box and a method for creating an RO-BDD that defines a configuration of the combinational circuit. The combinational circuit includes a number of independent selector groups for each generating an output bit separately, the number corresponding to number of the output bits, and a driver chain for supplying a primary input to each of the selector groups, in which each of the selector groups includes a plurality of selectors connected to each other to form a number of stages, the number of stages being equal to or less than number of bits of the primary input, and a select signal for the selectors in each stage is driven by one primary input.