Problem to be solved There is provided a method for detecting and/or amplifying a nucleic acid contained in a biological sample such as blood or cells conveniently, rapidly, and effectively.Solution There is provided a method for detecting a nucleic acid contained in a sample, comprising the step of adding at least one substance selected from the group consisting of polyphenols, polyhydric alcohols, sugar acids, sugar alcohols, and hydrophilic biodegradable polymers to a sample, the step of complementarily binding an oligonucleotide complementary to a part of the nucleic acid sequence of a nucleic acid to be detected to a part of the nucleic acid sequence, and the step of detecting the nucleic acid to be detected.