A method for detecting a polymorphism, the method comprising the steps of: amplifying a target sequence in a template nucleic acid in a reaction system containing primers and shown below, and detecting a polymorphism at a target site in the target sequence with a probe that can hybridize to the target sequence, wherein the target sequence comprises the target site showing the polymorphism, and a base (x) at the target site is either a first base or a second base, wherein: Primer is a primer comprising a sequence (A1') and a sequence, the sequence (A1') being complementary to a partial sequence in the template nucleic acid, and having, in its 3' region, a base (x1') complementary to the first base at the target site in the 5' region of the partial sequence, the sequence being noncomplementary to a partial sequence adjacent to the 3' end of the partial sequence in the template nucleic acid, and being bound to the 5' end of the sequence (A1'); and Primer is a primer comprising a sequence (A2'), the sequence (A2') being complementary to a partial sequence in the template nucleic acid, and having, in its 3' region, a base (x2') complementary to the second base at the target site in the 5' region of the partial sequence; wherein the number of bases in the sequence is more than 0 but not more than 5. amplifying a target sequence in a template nucleic acid in a reaction system containing primers and shown below, and detecting a polymorphism at a target site in the target sequence with a probe that can hybridize to the target sequence, wherein the target sequence comprises the target site showing the polymorphism, and a base (x) at the target site is either a first base or a second base, wherein: Primer is a primer comprising a sequence (A1') and a sequence, the sequence (A1') being complementary to a partial sequence in the template nucleic acid, and having, in its 3' region, a base (x1') complementary to the first base at the target site in the 5' region of the partial sequence, the sequence being noncomplementary to a partial sequence adjacent to the 3' end of the partial sequence in the template nucleic acid, and being bound to the 5' end of the sequence (A1'); and Primer is a primer comprising a sequence (A2'), the sequence (A2') being complementary to a partial sequence in the template nucleic acid, and having, in its 3' region, a base (x2') complementary to the second base at the target site in the 5' region of the partial sequence; wherein the number of bases in the sequence is more than 0 but not more than 5. Primer is a primer comprising a sequence (A1') and a sequence, the sequence (A1') being complementary to a partial sequence in the template nucleic acid, and having, in its 3' region, a base (x1') complementary to the first base at the target site in the 5' region of the partial sequence, the sequence being noncomplementary to a partial sequence adjacent to the 3' end of the partial sequence in the template nucleic acid, and being bound to the 5' end of the sequence (A1'); and a primer comprising a sequence (A1') and a sequence, the sequence (A1') being complementary to a partial sequence in the template nucleic acid, and having, in its 3' region, a base (x1') complementary to the first base at the target site in the 5' region of the partial sequence, the sequence being noncomplementary to a partial sequence adjacent to the 3' end of the partial sequence in the template nucleic acid, and being bound to the 5' end of the sequence (A1'); and Primer is a primer comprising a sequence (A2'), the sequence (A2') being complementary to a partial sequence in the template nucleic acid, and having, in its 3' region, a base (x2') complementary to the second base at the target site in the 5' region of the partial sequence; a primer comprising a sequence (A2'), the sequence (A2') being complementary to a partial sequence in the template nucleic acid, and having, in its 3' region, a base (x2') complementary to the second base at the target site in the 5' region of the partial sequence;