According to one embodiment, a method for detecting target nucleic acid includes the following steps. (A) A reaction field is formed by placing a reaction mixture on an electrode, and the reaction mixture contains the sample, a primer set, an amplification enzyme, 4 mM to 30 mM of magnesium ion, and a redox probe. The redox probe has an oxidation reduction potential, which generates an electric signal of which amplitude increases. (B) The reaction field is maintained under an amplification reaction condition. (C) The electric signal is detected with the electrode. (D) Existence or quantity of the target nucleic acid is determined.