The present invention relates to a method for detecting of a mutant DNA using a probe, comprising: (1) contacting a sample containing a single-stranded DNA which has a substituted nucleotide, a deleted nucleotide region, or an inserted nucleotide region (mutant-type DNA), or/and a wild-type single-stranded DNA (wild-type DNA) corresponding thereto with the probe which hybridizes with both single-stranded DNA, to form a hybrid with the mutant-type DNA (mutant-type hybrid) or/and a hybrid with a wild-type DNA (wild-type hybrid), wherein at least one of the obtained mutant-type hybrid and wild-type hybrid has the stem structure; (2) separating the obtained mutant-type hybrid or/and wild-type hybrid by electrophoresis on the basis of presence or absence of the stem structure or difference in the stem structure; and (3) detecting the presence or absence of the mutant-type DNA in the sample.