The present invention provides artificial universal base capable of base pairing nonspecifically with any of four kinds of natural nucleic acid bases (A, T, G, and C) without the function to specifically recognize pairing natural nucleic acid bases for base pair formation. Universal base capable of base paring nonspecifically with four kinds of natural nucleic acid bases, wherein the universal base has a structure represented by the following chemical formula: ##STR00001## wherein R represents a monovalent group other than a hydrogen atom.