The present invention addresses the problem of providing a novel, sold metal alloy. Provided is a metal alloy containing two or more types of metal, wherein an equilibrium diagram of the metal alloy shows the two or more types of metal in a finely mixed state at the nanolevel in a specific region where the two types of metal are unevenly distributed. This metal alloy has a substitutional solid solution of the two or more types of metal as the principal constituent thereof. This metal alloy is preferably one obtained by precipitation after mixing ions of two or more types of metal and a reducing agent in a thin-film fluid formed between processing surfaces, at least one of which rotates relative to the other, which are arranged so as to face one another and are capable of approaching and separating from one another.