It is intended to provide a thermostable AMP deaminase originating in a microorganism. Namely, an AMP deaminase having the following characteristics. (1) Catalyzing the reaction: 5′-adenylic acid+H2O→5′-inosinic acid+NH3; (2) being stable at a temperature of 65° C. or below (in an acetate buffer (pH 5.6)); (3) having a molecular weight of 48,000±2,000 in gel filtration; and (4) having the optimum pH value at around 5.6 (in McIlvaine buffer).