The present invention provides a primer set including primers that can be designed easily, with which an amplification distance can be shortened. Provided is a primer set for use in a method for isothermally amplifying a target nucleic acid sequence 4. The primer set includes a first primer 1F and a second primer 1R. The first primer 1F includes, on the 3′ side thereof, a sequence (A′) that can hybridize to a sequence (A) on the 3′ side of the target nucleic acid sequence. The second primer 1R includes, on the 3′ side thereof, a sequence (B′) that can hybridize to a sequence (B) on the 3′ side of either a strand extended from the first primer or a complementary strand of the target nucleic acid sequence 4. The first primer 1F and the second primer 1R include, on the 5′ sides thereof, sequences (C) that are substantially identical to each other.