A method for quantifying a target nucleic acid, comprising the following steps to: carrying out - an annealing reaction, - a nucleic acid extension reaction, and - a nuclease reaction by using as a template a single-stranded nucleic acid of target nucleic acid or a single-stranded nucleic acid obtained from a double-stranded nucleic acid of target nucleic acid that contains in direction from 3' end toward 5' end - a first sequence, - a sequence of interest and - a second sequence, along with a number of first primers that each contain in direction from 5' end toward 3' end a specific sequence, a random sequence and a sequence complementary to the first sequence and that each have a different sequence corresponding to the number of different sequences of the random sequence, - so as to obtain a first double-stranded nucleic acid with one strand containing in direction from 5' end toward 3' end - the specific sequence, - the random sequence, - the sequence complementary to the first sequence, - a sequence complementary to the sequence of interest, and - a sequence complementary to the second sequence; carrying out a PCR using the first double-stranded nucleic acid obtained from the step as a template along with a second primer containing the second sequence and a third primer containing the specific sequence so as to obtain a second double-stranded nucleic acid; determining the sequence of the second double-stranded nucleic acid obtained from the step; and measuring the number of combinations of the sequence of interest and the random sequence based on the sequence determined in the step and quantifying the target nucleic acid by the following equation: N = ln C − H C × − C wherein N represents the number of target nucleic acids; C represents the number of different sequences of the random sequence; and H represents the measured number of combinations of the sequence of interest and the random sequence. carrying out - an annealing reaction, - a nucleic acid extension reaction, and - a nuclease reaction by using as a template a single-stranded nucleic acid of target nucleic acid or a single-stranded nucleic acid obtained from a double-stranded nucleic acid of target nucleic acid that contains in direction from 3' end toward 5' end - a first sequence, - a sequence of interest and - a second sequence, along with a number of first primers that each contain in direction from 5' end toward 3' end a specific sequence, a random sequence and a sequence complementary to the first sequence and that each have a different sequence corresponding to the number of different sequences of the random sequence, - so as to obtain a first double-stranded nucleic acid with one strand containing in direction from 5' end toward 3' end - the specific sequence, - the random sequence, - the sequence complementary to the first sequence, - a sequence complementary to the sequence of interest, and - a sequence complementary to the second sequence; - an annealing reaction, - a nucleic acid extension reaction, and - a nuclease reaction by using as a template a single-stranded nucleic acid of target nucleic acid or a single-stranded nucleic acid obtained from a double-stranded nucleic acid of target nucleic acid that contains in direction from 3' end toward 5' end - a first sequence, - a sequence of interest and - a second sequence, along with a number of first primers that each contain in direction from 5' end toward 3' end a specific sequence, a random sequence and a sequence complementary to the first sequence and that each have a different sequence corresponding to the number of different sequences of the random sequence, - so as to obtain a first double-stranded nucleic acid with one strand containing in direction from 5' end toward 3' end - the specific sequence, - the random sequence, - the sequence complementary to the first sequence, - a sequence complementary to the sequence of interest, and - a sequence complementary to the second sequence; carrying out a PCR using the first double-stranded nucleic acid obtained from the step as a template along with a second primer containing the second sequence and a third primer containing the specific sequence so as to obtain a second double-stranded nucleic acid; determining the sequence of the second double-stranded nucleic acid obtained from the step; and measuring the number of combinations of the sequence of interest and the random sequence based on the sequence determined in the step and quantifying the target nucleic acid by the following equation: N = ln C − H C × − C wherein N represents the number of target nucleic acids; C represents the number of different sequences of the random sequence; and H represents the measured number of combinations of the sequence of interest and the random sequence.