A method for amplifying nucleic acids, comprising: step A including annealing a Bw adapter primer comprising the following structure (a): 5'-B1c-BL-B1-N3c-3'... (a) [Here, N3c represents an annealing region composed of a base sequence complementary to a base sequence on a 3'-end side of a target region of a target nucleic acid, BL represents a loop region comprising base sequence B2, and B1c and B1 represent stem regions comprising mutually complementary base sequences and capable of forming a double strand.] and the target region, synthesizing a base sequence complementary to a base sequence on a 5'-end side of the target region starting from a 3'-end of the Bw adapter primer, and obtaining a first template nucleic acid comprising the following structure (b): 5'-B1c-BL-B1-N3c-N5c-3'... (b) [Here, N5c represents the base sequence complementary to the base sequence on the 5'-end side of the target region, and a base sequence comprising N3c and N5c represents base sequence Nc complementary to the target region.] in which a stem-loop structure is added to a 5'-end of a complementary strand of the target region; step B including annealing an Fw adapter nucleotide comprising the following structure (c): 5'-F1c-FL-F1-N5'-3'... (c) [Here, N5' represents an annealing region composed of a base sequence complementary to base sequence N5c of the first template nucleic acid, FL represents a loop region comprising base sequence F2, and F1c and F1 represent stem regions comprising mutually complementary base sequences and capable of forming a double strand.] and having an extension-inhibiting modification at a 3'-end and the first template nucleic acid, synthesizing a complementary strand of the Fw adapter nucleotide starting from a 3'-end of the first template nucleic acid, and obtaining a second template nucleic acid comprising the following structure (d): 5'-B1c-BL-B1-N3c-N5c-F1c-FLc-F1-3'... (d) [Here, FLc represents a base sequence complementary to FL of the Fw adapter nucleotide.] in which a stem-loop structure is added to a 3'-end of the first template nucleic acid; and step C including amplifying base sequence Nc of the second template nucleic acid by a LAMP method by using an Fw inner primer comprising the following structure (e): 5'-F1c-F2-3'... (e) and a Bw inner primer comprising the following structure (f): 5'-B1c-B2-3'... (f) with the second template nucleic acid as a template. step A including annealing a Bw adapter primer comprising the following structure (a): 5'-B1c-BL-B1-N3c-3'... (a) [Here, N3c represents an annealing region composed of a base sequence complementary to a base sequence on a 3'-end side of a target region of a target nucleic acid, BL represents a loop region comprising base sequence B2, and B1c and B1 represent stem regions comprising mutually complementary base sequences and capable of forming a double strand.] and the target region, synthesizing a base sequence complementary to a base sequence on a 5'-end side of the target region starting from a 3'-end of the Bw adapter primer, and obtaining a first template nucleic acid comprising the following structure (b): 5'-B1c-BL-B1-N3c-N5c-3'... (b) [Here, N5c represents the base sequence complementary to the base sequence on the 5'-end side of the target region, and a base sequence comprising N3c and N5c represents base sequence Nc complementary to the target region.] in which a stem-loop structure is added to a 5'-end of a complementary strand of the target region; step B including annealing an Fw adapter nucleotide comprising the following structure (c): 5'-F1c-FL-F1-N5'-3'... (c) [Here, N5' represents an annealing region composed of a base sequence complementary to base sequence N5c of the first template nucleic acid, FL represents a loop region comprising base sequence F2, and F1c and F1 represent stem regions comprising mutually complementary base sequences and capable of forming a double strand.] and having an extension-inhibiting modification at a 3'-end and the first template nucleic acid, synthesizing a complementary strand of the Fw adapter nucleotide starting from a 3'-end of the first template nucleic acid, and obtaining a second template nucleic acid comprising the following structure (d): 5'-B1c-BL-B1-N3c-N5c-F1c-FLc-F1-3'... (d) [Here, FLc represents a base sequence complementary to FL of the Fw adapter nucleotide.] in which a stem-loop structure is added to a 3'-end of the first template nucleic acid; and step C including amplifying base sequence Nc of the second template nucleic acid by a LAMP method by using an Fw inner primer comprising the following structure (e): 5'-F1c-F2-3'... (e) and a Bw inner primer comprising the following structure (f): 5'-B1c-B2-3'... (f) with the second template nucleic acid as a template.