A method of detecting a target base sequence having a polymorphic base comprising (a) a step of adding the following primers to a nucleic acid sample having a target nucleic acid that comprises a base sequence including the target base sequence: at least one type of detection primer that is complementary to said target base sequence, at least one type of competitive primer that is complementary to said target base sequence as well as being capable of annealing to a target nucleic acid, in a competitive manner against said detection primer, and at least one type of common primer, (b) a step of annealing said detection primer and said competitive primer to said target nucleic acid in a competitive manner with use of the target base sequence having the polymorphic base in said nucleic acid sample as a template, thereby causing an extension reaction to synthesize an extension product A, (c) a step of annealing said common primer to said extension product A obtained in said step (b) or in the following step (d), thereby causing an extension reaction to synthesize an extension product B, (d) a step of annealing said detection primer or said competitive primer to the extension product B obtained in said step (c), thereby synthesizing the extension product A, and (e) a step of detecting the extension product A or the extension product B, wherein said detection primer has a match base that is complementary to the polymorphic base of the target sequence, and has at least one first mismatch base that is not complementary to a base of the target sequence other than the polymorphic base; said competitive primer has a mismatch base that is not complementary to the polymorphic base of the target sequence, and has at least one first mismatch base and optionally at least one second mismatch base that are not complementary to a base of the target sequence other than the polymorphic base; the position of the at least one first mismatch base of said detection primer is different from the position of the at least one first mismatch base of said competitive primer that is not complementary to the base of the target sequence other than the polymorphic base; the detection primer has the match base that is complementary to the polymorphic base of the target sequence, at the 3' end or the second base from the 3' end; said common primer is capable of making a primer pair with said detection primer or said competitive primer to amplify said target nucleic acid; a difference in a chain length between said competitive primer and said detection primer is within 16 bases; when present, the second mismatch base of said competitive primer is located 7 bases or more away from the 3' end to the 5' side. (a) a step of adding the following primers to a nucleic acid sample having a target nucleic acid that comprises a base sequence including the target base sequence: at least one type of detection primer that is complementary to said target base sequence, at least one type of competitive primer that is complementary to said target base sequence as well as being capable of annealing to a target nucleic acid, in a competitive manner against said detection primer, and at least one type of common primer, at least one type of detection primer that is complementary to said target base sequence, at least one type of competitive primer that is complementary to said target base sequence as well as being capable of annealing to a target nucleic acid, in a competitive manner against said detection primer, and at least one type of common primer, (b) a step of annealing said detection primer and said competitive primer to said target nucleic acid in a competitive manner with use of the target base sequence having the polymorphic base in said nucleic acid sample as a template, thereby causing an extension reaction to synthesize an extension product A, (c) a step of annealing said common primer to said extension product A obtained in said step (b) or in the following step (d), thereby causing an extension reaction to synthesize an extension product B, (d) a step of annealing said detection primer or said competitive primer to the extension product B obtained in said step (c), thereby synthesizing the extension product A, and (e) a step of detecting the extension product A or the extension product B, wherein