According to one embodiment, a primer set for elongating a target short-chain nucleic acid containing a first sequence to obtain an elongated product is provided. The elongated product contains a second, a third, a fourth sequence, a complementary sequence of the 1′-th sequence and a sixth sequence. The complementary sequence of the 1′-th sequence is a loop primer sequence. The primer set contains a first elongation primer containing a first elongation primer sequence and a complementary sequence of the sixth sequence, and a second elongation primer containing a second elongation primer sequence, the fourth, the third, and the second sequence.