A nucleic acid amplification method which comprises performing isothermal incubation at a temperature of 50°C to 100°C of a reaction solution containing at least one type of deoxynucleotide triphosphate, at least one type of DNA polymerase having strand displacement activity, a divalent cation, at least 0.01% or more nonionic surfactant having an HLB value of 14 or more, at least two types of oligonucleotide primer wherein at least one of the two types of oligonucleotide primers is a forward primer and at least one of the two types of oligonucleotide primers is a reverse primer, wherein the reaction solution contains a 1μM to 50μM oligonucleotide primer, and the nucleic acid fragment as a template so as to perform a polymerase reaction that initiates from the 3' end of the primer and thus amplifying the nucleic acid fragment, wherein the primers do not have a structure which forms a loop structure wherein 5' terminal is complementary to the region which was elongated from the 3' terminal, and do not have a system where the primer is cleaved during the reaction and the cleaved 3' terminal serves as a synthesis origin, wherein (a) one of at least two existing types of primer is annealed to a template nucleic acid strand and a synthesis reaction is initiated from the 3' end with the use of the strand displacement-type DNA polymerase so as to synthesize an elongation product; (b) oligonucleotide primers of a different type than the primers used in step (a) are caused to enter the double stranded nucleic acid obtained in the above step, without performance of any denaturation of the double strands via application of a temperature higher than that in step (a), initiating a synthesis reaction from the 3' end by the action of the strand displacement-type DNA polymerase so as to synthesize an elongation product; (c) the same primers as those used in step (a) are annealed again to the nucleic acid liberated in step (b) above and a synthesis reaction is then initiated from the 3' end by the action of at least one type of strand displacement-type DNA polymerase, so as to synthesize an elongation product; and then (d) the double stranded nucleic acid obtained in step (b) above is used again in step (b). (a) one of at least two existing types of primer is annealed to a template nucleic acid strand and a synthesis reaction is initiated from the 3' end with the use of the strand displacement-type DNA polymerase so as to synthesize an elongation product; (b) oligonucleotide primers of a different type than the primers used in step (a) are caused to enter the double stranded nucleic acid obtained in the above step, without performance of any denaturation of the double strands via application of a temperature higher than that in step (a), initiating a synthesis reaction from the 3' end by the action of the strand displacement-type DNA polymerase so as to synthesize an elongation product; (c) the same primers as those used in step (a) are annealed again to the nucleic acid liberated in step (b) above and a synthesis reaction is then initiated from the 3' end by the action of at least one type of strand displacement-type DNA polymerase, so as to synthesize an elongation product; and then (d) the double stranded nucleic acid obtained in step (b) above is used again in step (b).