To provide a Cu-based amorphous alloy having a glass-forming ability higher than that of a Cu--Zr--Ti amorphous alloy and a Cu--Hf--Ti amorphous alloy, as well as excellent workability and excellent mechanical properties without containing large amounts of Ti.A Cu-based amorphous alloy characterized by containing 90 percent by volume or more of amorphous phase having a composition represented by Formula: Cu.sub.100-a-b(Zr,Hf).sub.a(Al,Ga).sub.b [in Formula, a and b are on an atomic percent basis and satisfy 35 atomic percent.ltoreq.a.ltoreq.50 atomic percent and 2 atomic percent.ltoreq.b.ltoreq.10 atomic percent], wherein the temperature interval .DELTA.Tx of supercooled liquid region is 45 K or more, the temperature interval being represented by Formula .DELTA.Tx=Tx-Tg (where Tx represents a crystallization initiation temperature and Tg represents a glass transition temperature.), a rod or a sheet having a diameter or thickness of 1 mm or more and a volume fraction of amorphous phase of 90% or more can be produced by a metal mold casting method, the compressive strength is 1,900 MPa or more, the Young's modulus is 100 GPa or more, and the Vickers hardness is 500 Hv or more.