A semi-nested polymerase chain reaction (PCR) was introduced to amplification of the HLA-DQA1 gene, which is a single-copy gene, in a single genome. The limitation of template DNA for genotyping is 1 ng of genomic DNA, or more in the case of ordinary PCR. When reamplification with the same primers was performed, primer dimer was generated and the sensitivity was not improved. We designed a semi-nested primer for the second round of PCR. using the semi-nested PCR, more than 3 pg of template DNA could be amplified and typed. Furthermore, this method was applied to amplify DQA1 gene in single human sperm having haploid DNA, and followed by typing with sequence-specific oligonucleotide (SSO) probes. The semi-nested PCR technique was found to enhance the sensitivity of the amplification reaction and allowed the successful typing of the HLA-DQA1 gene. This is helpful for genotyping from samples with extremely small amounts of DNA, such as forensic or ancient DNA samples.