A method for determining whether or not an unmethylated cytosine conversion treatment that converts unmethylated cytosine into a base other than cytosine is properly carried out, comprising the steps of: converting unmethylated cytosine of the biological DNA contained in a sample into a base other than cytosine, to give an unmethylated cytosine conversion sample; carrying out nucleic acid amplification reactions of the following (i) and (ii): (i) a nucleic acid amplification reaction using the unmethylated cytosine conversion sample obtained in the step and a first primer set comprising a primer that hybridizes with a nucleotide sequence of the biological DNA, said nucleotide sequence not containing cytosine and (ii) a nucleic acid amplification reaction using the unmethylated cytosine conversion sample obtained in the step and a second primer set comprising plural primers that hybridize with a nucleotide sequence in which cytosine in the nucleotide sequence containing cytosine and not containing a CpG site is converted into a base other than cytosine in the nucleotide sequence of the biological DNA; wherein the nucleotide sequence not containing cytosine and the nucleotide sequence containing cytosine and not containing a CpG site are nucleotide sequences present in the same chromosome of the biological body, determining the amount of the amplification product obtained in the nucleic acid amplification reaction (i) and the amount of the amplification product obtained in the nucleic acid amplification reaction (ii) of the step; calculating the ratio of the nucleic acid in which the unmethylated cytosine of the biological DNA is converted into a base other than cytosine in total nucleic acids contained in the unmethylated cytosine conversion sample obtained in the step, based on the determination result obtained in the step; and determining whether or not the step is properly carried out, based on the calculation result obtained in the step. converting unmethylated cytosine of the biological DNA contained in a sample into a base other than cytosine, to give an unmethylated cytosine conversion sample; carrying out nucleic acid amplification reactions of the following (i) and (ii): (i) a nucleic acid amplification reaction using the unmethylated cytosine conversion sample obtained in the step and a first primer set comprising a primer that hybridizes with a nucleotide sequence of the biological DNA, said nucleotide sequence not containing cytosine and (ii) a nucleic acid amplification reaction using the unmethylated cytosine conversion sample obtained in the step and a second primer set comprising plural primers that hybridize with a nucleotide sequence in which cytosine in the nucleotide sequence containing cytosine and not containing a CpG site is converted into a base other than cytosine in the nucleotide sequence of the biological DNA; wherein the nucleotide sequence not containing cytosine and the nucleotide sequence containing cytosine and not containing a CpG site are nucleotide sequences present in the same chromosome of the biological body, (i) a nucleic acid amplification reaction using the unmethylated cytosine conversion sample obtained in the step and a first primer set comprising a primer that hybridizes with a nucleotide sequence of the biological DNA, said nucleotide sequence not containing cytosine and (ii) a nucleic acid amplification reaction using the unmethylated cytosine conversion sample obtained in the step and a second primer set comprising plural primers that hybridize with a nucleotide sequence in which cytosine in the nucleotide sequence containing cytosine and not containing a CpG site is converted into a base other than cytosine in the nucleotide sequence of the biological DNA; wherein the nucleotide sequence not containing cytosine and the nucleotide sequence containing cytosine and not containing a CpG site are nucleotide sequences present in the same chromosome of the biological body, determining the amount of the amplification product obtained in the nucleic acid amplification reaction (i) and the amount of the amplification product obtained in the nucleic acid amplification reaction (ii) of the step; calculating the ratio of the nucleic acid in which the unmethylated cytosine of the biological DNA is converted into a base other than cytosine in total nucleic acids contained in the unmethylated cytosine conversion sample obtained in the step, based on the determination result obtained in the step; and determining whether or not the step is properly carried out, based on the calculation result obtained in the step.