An asynchronous adder permits asynchronous design in which dual-rail encoding is employed, not only for a control part but also for a datapath part including an ALU. An asynchronous adder of an exemplary embodiment includes a combinational circuit to perform full addition with, as an input value, an addend X, an augend Y and a carry-in C.sub.in that are dual-rail encoded, and to output a sum output Z and a carry output C.sub.out that are dual-rail encoded as an output value.