An aluminum base alloy is produced by supercooling a molten alloy composed mainly of aluminum. The molten alloy contains an element capable of forming a quasicrystalline phase, an element which aids formation of the quasicrystals, and an element which stabilizes a supercooled state of the molten alloy and delays crystallization of a crystalline phase, and is composed of a mixed composition of a fine amorphous phase and an aluminum crystalline phase or an aluminum supersaturated solid solution phase, or a single phase of only an amorphous phase.