A method of distinguishing genotypes of a gene mutation comprising: a nucleic acid amplification step in which a mutation site-including region of a gene contained in a specimen is amplified by a nucleic acid amplification reaction, thereby obtaining an amplification reaction solution which comprises a specimen double-stranded nucleic acid; and a distinction step in which the amplification reaction solution obtained from the nucleic acid amplification step is mixed with a reference double-stranded nucleic acid having a specific genotype on the mutation site as well as being labeled with a labeling substance, and the mixture is subjected to a competitive strand displacement reaction, and a level of an occurrence of strand displacement between the reference double-stranded nucleic acid and the specimen double-stranded nucleic acid is assessed so as to distinguish an identity between the reference double-stranded nucleic acid and the specimen double-stranded nucleic acid, wherein the competitive strand displacement reaction is performed under a condition to suppress a polymerase extension reaction; and the condition comprises performing the competitive strand displacement reaction under the presence of a chelating agent, subjecting the amplification reaction solution obtained from the nucleic acid amplification step to a heat treatment at 95°C or higher temperature for 10 minutes or longer prior to the competitive strand displacement reaction, or subjecting the amplification reaction solution obtained from the nucleic acid amplification step to a single-stranded nucleic acid degradation treatment or a nucleotide triphosphate degradation treatment by an exonuclease I, an exonuclease T, a mung bean nuclease or apyrase prior to the competitive strand displacement reaction. a nucleic acid amplification step in which a mutation site-including region of a gene contained in a specimen is amplified by a nucleic acid amplification reaction, thereby obtaining an amplification reaction solution which comprises a specimen double-stranded nucleic acid; and a distinction step in which the amplification reaction solution obtained from the nucleic acid amplification step is mixed with a reference double-stranded nucleic acid having a specific genotype on the mutation site as well as being labeled with a labeling substance, and the mixture is subjected to a competitive strand displacement reaction, and a level of an occurrence of strand displacement between the reference double-stranded nucleic acid and the specimen double-stranded nucleic acid is assessed so as to distinguish an identity between the reference double-stranded nucleic acid and the specimen double-stranded nucleic acid, wherein the competitive strand displacement reaction is performed under a condition to suppress a polymerase extension reaction; and the condition comprises performing the competitive strand displacement reaction under the presence of a chelating agent, subjecting the amplification reaction solution obtained from the nucleic acid amplification step to a heat treatment at 95°C or higher temperature for 10 minutes or longer prior to the competitive strand displacement reaction, or subjecting the amplification reaction solution obtained from the nucleic acid amplification step to a single-stranded nucleic acid degradation treatment or a nucleotide triphosphate degradation treatment by an exonuclease I, an exonuclease T, a mung bean nuclease or apyrase prior to the competitive strand displacement reaction.