The invention relates to a mutant .alpha.-amylase obtained by making replacement or deletion of at least one of amino acid residues such as the 167th Gln, 169th Tyr and 178th Ala in the amino acid sequence set forth in SEQ ID NO:1 in an .alpha.-amylase having said amino acid sequence, or an .alpha.-amylase having a homology of at least 70% to said amino acid sequence, a gene encoding the mutant .alpha.-amylase, a vector, transformed cells, a process for producing a mutant .alpha.-amylase, comprising culturing the transformed cells, and a detergent composition comprising the mutant .alpha.-amylase.The mutant .alpha.-amylase of the invention has excellent properties of high resistance to chelating agents, high specific activity in an alkaline region and excellent stability to heat, and is hence useful for detergents for automatic dish washer, laundry detergents and the like.