[Problems] To provide a novel mixture for assaying a target nucleic acid, characterized by enabling a nucleic acid assay while: 1) requiring no step of diluting the target nucleic acid; 2) requiring no procedure of changing a probe concentration depending on a concentration of the target nucleic acid.[Means for Solving Problems] 1) A mixture which comprises one internal standard nucleic acid and two nucleic acid probes labeled with a fluorescent dye; 2) a mixture for measuring Km value which comprises one internal standard nucleic acid having a partial mutation and one nucleic acid probe labeled with a fluorescent dye; 3) a mixture which comprises one internal standard nucleic acid and one double nucleic acid probe labeled with two fluorescent dyes; and a method for assaying a nucleic acid by making use thereof.